AITA for Exploding at My Brother-in-Law After He Cheated on My Sister During Our Joint Vacation?

My sister and brother-in-law (both in their mid-30s) have two small kids—a 6-year-old and an 8-month-old. I’m a 40-year-old man who lived with my sister when she began dating him, and I even served as best man at their wedding (which felt a little strange to me, but I was flattered they asked).

We all shared a home until the older child was around 3, when they decided to get their own space. In total, we probably cohabited for about 5 years with only normal roommate irritations and typical sibling disagreements. I’ve remained tight with them since they moved out, visiting frequently and staying very involved in the older child’s life. I’ve also helped watch the baby on occasion.

I was out of work for a while before returning to an old job. Once I knew I had the position lined up, I casually mentioned wanting a quick trip before starting. My BIL immediately latched onto the idea and pushed hard for an all-inclusive getaway with just the two of us. I knew it was probably a mistake, but he was enthusiastic and I went along with it. My sister seemed okay with the plan but set two clear conditions: we had to share a room and there would be no drugs.

Before their marriage, they had split up for a period because he cheated. I half-jokingly warned my sister that “something is going to happen and I’m going to end up the villain.” I even pulled him aside on the way to the airport and asked him not to put me in an awkward position. He seemed surprised but agreed.
On our first full day there, he wandered off to “check out” another resort bar with a random woman we had just met. After he was gone for over an hour, I went looking for him at the bar but he wasn’t there. When I returned to our room, he was doing cocaine with her inside. I thought it was odd, but figured we were on vacation. We partied throughout the day.

That night at a bar, I started chatting with another woman. My BIL and I have a somewhat competitive dynamic, and when he drinks he has a habit of inserting himself if I’m talking to someone. Eventually he and this woman were getting very cozy, disappearing together to smoke, and behaving like flirty teenagers—dancing, flirting, and getting a bit physical. I told him it looked weird and he dismissed it, claiming I was imagining things and that he was just trying to hook me up. I was intoxicated and messed up, so I dropped it.

Later I went back toward the room with another woman, but my key wouldn’t work. This was the same person he had been using substances with earlier, and they had swapped numbers. We both tried contacting him but he left us on read. I eventually had to get a new key from the front desk and ended up sleeping alone. This was around 1 a.m.

I woke up at 7 a.m. and he still wasn’t back. I texted him with no reply, then went searching. I found him at the closed pool with the woman from the bar while it was still dark and no one else was around. He greeted me casually like nothing was wrong.
I lost it on him right there. She hid under her towel while I unloaded. He claimed nothing happened, said he saw me head back with the other woman and wanted to give me space, so he slept on the beach. Supposedly she had just run into him at the pool recently.

I didn’t want drama for the rest of the week, so I accepted it but was furious inside. Later I ran into people from the night before who told me they had seen him making out with her, going down to the beach together, and more. I confronted him again. He admitted they went to the beach but insisted nothing “serious” occurred. He said she wanted to have sex but he refused because of his family, then she left and came back to talk at the pool around 6:30 a.m.

I told him his story was nonsense and he needed to confess to my sister. He apologized but kept minimizing it. I spent the day on my own, and when I returned he said he had told her. She was upset but they planned to fix things at home. I should have called her myself but didn’t, and said I’d let it go.

I didn’t let it go. I made digs at him all week. A few nights later, heavily drunk, I completely unloaded—called him a piece of garbage, insulted him, and compared him to his deadbeat dad who abandoned him as a child. I was filled with raw anger. I texted my sister saying I hated him and that while she might forgive, I wasn’t sure I could.
The next morning she texted asking what she was supposed to forgive him for. When I called, she said he told her I fell for lies from women trying to stir trouble, that nothing happened, and I only saw him at the pool with her. I shared my version (at least part of it) and she said “he’s my husband, I have to believe him” while sounding choked up. She added that he now hates me and wants nothing to do with me.

He demanded she pick him up from the airport two hours away because he couldn’t stand riding with me. He also said if I thought such terrible things about him, he didn’t want me around the kids. My sister was distraught and said we’d discuss it when we got home.

I later apologized to him for going too far with the personal attacks, but he refused to accept it. We acted like strangers for the rest of the trip. My sister had to pay a friend $200 to pick him up because she couldn’t manage it with two young children.
It’s been nearly three months. I haven’t seen the kids, who I used to see 2-3 times a week. He still despises me. My sister says they’re in therapy, he knows he’s being unreasonable but is enforcing a boundary “for now.” She says he gets furious if she mentions it and doubles down, but claims it won’t be permanent.
I’ve struggled with drinking. I’ve had blow-ups with my other sister and a past drunken incident with my mom. He’s fixated on this, saying I need to get sober and that everyone makes excuses for me. This was part of why we eventually split living spaces, but I’ve always felt what I do in my private time is my business.
The past arguments with my sisters were about my drinking and me telling them to stay out of it. My BIL was never involved before. I was asked not to be intoxicated around the kids previously, and I respected that completely. I babysat often, hosted the older child, and picked him up from school. We were close.
I haven’t had a drink since we returned. This situation was the final push; I had already been thinking about quitting. Nothing positive ever comes from it.

My family is mostly staying neutral, but my mom is upset because we’re usually very close. This past Easter was the first time we didn’t gather as a family because he gave my sister an ultimatum and stayed home with the kids.
My sister initially told my mom it was all my fault right after the trip. My dad spoke to my BIL, who said he just “needs time.” My sister has been apologetic to me and normal in the family chat, but she’s choosing to believe him to avoid divorce. She knows the truth deep down.
I shared extra details he omitted, and she believes me but still says she “has to” believe him that nothing happened. They needed a sitter on Mother’s Day because the kids couldn’t go to my parents’ if I was there.
That was the first time I saw her since the trip. We talked briefly and planned a dinner this week to discuss things privately. She’s worried our parents hate him (they do, mostly over the current fallout). She’s framing it to them as being about how I treated him afterward, not what he did.
I haven’t contacted him or pushed my sister much, giving them space. I did tell my parents the truth—I won’t cover for him. He changed the door codes right after getting home. I would never barge in uninvited. It’s ridiculous.
Is this insane or am I the crazy one? Am I the asshole? Do I deserve to be cut off from the kids because I was harsh with him? I swing between rage at the whole mess and guilt for the nasty things I said.
Here’s what people had to say to OP:
(Paraphrased comments maintaining original tone and key points)
iknowsomethings2: He cheated on your sister. She’s being weak by not setting firm boundaries. If he’ll cheat on a trip with her own brother, he’ll do it anywhere. Step back and let her know you’ll support her when she eventually leaves after the next time. That’s all you can do. Why stay friends with him now? Your parents should stay involved or he’ll isolate her and treat her poorly.
BasicRabbit4: Honestly, whether he slept with them doesn’t even matter. His behavior on vacation was totally wrong for a married man. You don’t spend all night flirting, disappearing, and acting couple-like if you’re committed. He wants you gone so he can keep lying without pushback. This probably isn’t his first time.
Midi58076: I agree, but maybe he suggested the trip with you because of your drinking issues. A known drinker is easy to discredit, yet also less likely to stop wild behavior.

Ambitious-Bat237: You both seem like irresponsible jerks. Your sister knows the truth but is choosing denial. Hopefully this wakes you up about your drinking.
Puzzleheaded_Cod1181: NTA. You did the right thing. Your sister is falling for his gaslighting. Your BIL is trash. She’ll see it eventually. Just be ready to support her when she leaves.
NotUniqueScott: You don’t deserve to be cut off, but this shows actions have consequences. Sometimes doing the right thing costs you. No good deed goes unpunished. Your sister is in denial but made a practical choice. Hope this helps you get sober.
According_Pizza8484: Your sister is burying her head in the sand. Until she leaves, there’s little you can do. Your family seems to prioritize peace, which lets him shift blame to your behavior instead of his cheating. Getting sober was the best move. Stop apologizing for telling the truth—it gives him undeserved power. Tell her you’ll be there if she leaves, but you won’t keep begging forgiveness. If the family enables him, take space for your sobriety. Maybe later when the kids are older things can improve.
Facebook Description:
AITA for exploding at my BIL after catching him cheating on my sister during our guys’ vacation? Family drama explodes when he gaslights everyone and cuts OP off from the kids. Brutally honest Reddit opinions inside—whose side are you on?

My thoughts and advice for OP:
This is a classic messy family situation where everyone is flawed but the BIL bears the primary fault. You’re not the asshole for confronting him—his behavior was blatantly inappropriate and disrespectful to your sister. The heavy personal insults and drunk outburst crossed a line, which gave him ammunition to flip the script and play victim. That said, his refusal to accept a reasonable apology and weaponizing access to the kids is manipulative and punitive.
Advice:

Stay sober. That’s the strongest move you can make right now for yourself and your credibility.
Stop apologizing to him. You’ve done it once; further groveling gives him power.
Support your sister without badmouthing her husband constantly. Let her know you’ll be there when (not if) things fall apart, but respect her current position.
Document interactions calmly and keep the door open for the kids through your sister/parents when possible.
Let your parents see the full picture without forcing a family blow-up.

Focus on your own healing and sobriety. The truth tends to surface eventually.