Most Helpful Guy
A lot of these 'cheating male' stories sound horrid. The guy comes across as illogical and thoughtless. As a kid, I felt the same way about my dad...
On growing up, I know better and see the other side of the story.
It's usually something like this (ask just about any guy):
1. Girl falls for guy.
2. Gives him great sex, she can't seem to have enough.
3. He falls for her, is willing to sacrifice and do what she wants.
4. They get into a relationship, or marry.
5. They have kids (which accelerates the crisis).
6. She loses interest in sex.
7. The sex tap dries up.
8. He finds it elsewhere.
I think sex is a crucial part of a relationship/marriage, but many are not willing to acknowledge this. This is where most of the problems start, when the wife/girlfriend starts to think she's caught a guy, and is not obliged to bother too much about sex.
(Actually, she can't help it. Because women are also made that way and tend to lose their interest in a long-term partner. Some very good explanations of the two-year-itch and women!)
Most Helpful Girl
You're telling one side of the story, I'm sure if your brother-in-law could tell his it would be different. You are siding with your sister, because she's your sister, you're not looking at the situation without bias. The first thing that is more important than anything is your nephew, hopefully your brother -in-law is taking good care of him, since your sister cannot. Your sister screwed up big time, by caring more about going out to bars and screwing random guys, than taking care of her son, you can't blame your brother-in-law for that. You are blaming everything on your brother-in-law and that's not fair, he isn't 100% to blame, he is 50% to blame. Your sister is just as much to blame. Their marriage is over, your brother-in-law has your nephew, so you need to keep your feelings to yourself about your brother-in-law, and never talk poorly about him in front of his son. Your sister's marriage is really none of your business tbh, just help her move on, it's clearly over between them.