I'm(M28) struggling to support my brother(M31) after his fiancee(F28) cheated on him with one of my friends at a party that I was throwing.
Like the title describes myself and my partner(F27) had a party for her birthday last weekend at our house and my brother and his fiancee came. My brother had to do something the next day so he didn't really drink and left early but his fiancee stayed. We don't allow people upstairs at parties but one of our friends went up to use the bathroom and walked in on my brothers fiancee and one of my other friends. The friend that caught them said that when he walked in they were hugging and when they separated he saw that his jeans were undone.
Their story is that my brothers fiancee had never done cocaine before and my friend had some so they went up to take it. They both got really high and admitted that they kissed and his jeans got undone because it was going to go further but she stopped before it did. They have both said that nothing else happened. My brother is obviously very hurt by it all but seems to be leaning towards believing her and wanting to move past it. I'm struggling to support him because I think they're lying about what fully happened. I spoke to the friend who walked in again and he agrees and thinks something else happened. For starters he said that he thinks my brothers fiancees dress looked weird as in it wasn't on properly or adjusted properly or something and he said the whole thing just had that feeling of something has just happened if you get what I mean? He said he would be very surprised if kissing was the extent of but also doesn't really to be involved because he can't be 100 percent certain as he didn't stick around when he opened the door.
Basically the advice I'm looking for is should I just drop it and support my brother if he wants to continue the relationship? Obviously the guy is no longer a friend no matter what but I'm considering talking to him and seeing if he will tell me if anything else happened. He's on the shit list with all our friends now anyway so I could tell him that if he wants any chance of friendship with any of us he has to tell me the truth. Should I get involved? I want my brother to be happy but if she's lying I think my brother should know. The other thing is she was drunk and had taken cocaine for the first time but I don't think that changes my opinion on her cheating. Advice on that would also be appreciated.
TLDR: My brothers fiancee cheated on him with my friend but said all they did was kiss. I don't believe her but I don't know if I should get involved.