I really need advice...LONG

So I have been with my SO for about two years now and I am really close to his family.

My BIL has been with his son's mother for about four years. He is 21, and she is 19. They had their son when she was 17 and he was 19, their son is now two.
For the last six months they have been off and on because they had been having problems or whatever.

I personally cannot stand her and we have had a lot of issues but my in-laws won't allow us to fight in their house or anything so we basically just ignore each other while we're there.

Recently, my BIL has gotten into a relationship with another girl and she is really nice and friendly, the total opposite of the BM. They were together for like a month then he started to stay there with her and they were together everyday basically living together. He had told us before that he really loves her and never wants to get back with his BM. The BM was out of town with their son going to school. Then she just quit school and came back for Halloween weekend to stay at our in-laws so my BIL could spend time with his son.

So my BIL comes back from his gf's house back to his parent's and as soon as he sees his BM again she starts getting him to kiss her, hold her, etc. And they look like they are right back together again and are sleeping together in his room on his bed!

So apparently the gf knows NOTHING about any of this he is telling her that nothing is going on between them that everytime she tries something he walks away, which is a TOTAL lie because I am there a lot and have seen all of this!
He also told her he is sleeping in his brother's room which is also a lie!

Like two hours ago she made a comment on my status on MySpace and I asked her how she was and she just started telling me all of these things about how the BM is texting her talking shit to her saying they had sex like 10x and they are back together...

So I text her and ask her what's going on and she tells me all that about what he's been telling her...and since I know he's lying I tell her. I asked her please not to tell him I said anything because I can't get brought into it.

Now I feel REALLY bad about telling her because they are like my family you know and my SO would be really mad if he knew I said anything because that's his brother and I'm like ratting on him or whatever. And it's not like this girl has been in my life for a long time I barely know her. It's really none of my business but I feel so bad that she is just in the dark about what's going on and I had to get it off my chest...

Now the BM is saying she's going to live there at his parents and the gf doesn't know what to do because she can't tell him "I don't want her living there or come back here" because then the BM will just take his son away. She is very vindictive like that and if she doesn't get her way she will punish him by not letting him see his son and that is the MOST important thing to him.

So did I do the right thing? I really don't know. I love my BIL but it's not right for him to have his cake and eat it too. I just don't know...I feel so guilty.

I would have done the same thing. No one deserves to be cheated on.

I would have done the same thing, hun.

Ive been in that situation...Its hard cause its your family thats fuckin up....and good people get involved...I didnt do what you did though..I thought about it long and hard..and just figured if they wanted to know..I'd tell them, but Im not getting in that kinda buisness...Im sorry you had to make that desicion 

What should I tell my BIL or my SO if they find out it was me that told her? 

Quoting A.Trevino:“ What should I tell my BIL or my SO if they find out it was me that told her? ”

Tell them the truth. You told her because you think she has the right to know what was going on.

Quoting A.Trevino:“ What should I tell my BIL or my SO if they find out it was me that told her? ”

If it were me, I'd tell them "Yes, I told her. It's not right what you're (he's *if you're talking to your SO) doing to her and I'm not going to sit here and watch her get hurt while you screw her over with the cunt of a babymomma you've (or He's *if you're talking to your so) got."

That's just me though. lol.

Quoting A.Trevino:“ What should I tell my BIL or my SO if they find out it was me that told her? ”

Tell 'em the truth....You didnt do shit wrong...hes the one who couldnt be faithful...

Thanks ladies. I guess I feel better about it now. Just hoping things go smoothly and I'm not brought into it. If not, I guess I will just be blunt and tell the truth.