<human>: Can you provide a brief overview of the text?

Throw away for reasons, I don't want them connected to my other account. 
So I live with my brother and his girlfriend. They have been dating for years and have a 8 month old baby together. 

I suspected of her maybe falling out of love with my brother; They argue a lot. She finds support with any guy friend that will talk to her. She's been having a friend come over a lot, let's call him Sam. My brother trusted her because she just has more guy friends than girls. I just thought they were a little too close, and she talks about him a lot but I ignored it, thinking I worry too much. 

I walked in on them on the floor, clothed, her on top, making out with Sam. They didn't notice me, I moved back and dropped my phone a few feet back and around the corner of the room and acted normal till my friend picked me up. I went and got my brother after work and told him. He's a very passive guy. He just said okay and started texting. 

Here is a women who doesn't work, spends his hard working money on anything and everything and now is cheating on him. Sam has been coming over for a long time, sleeping here and eating food my brother he pays for. 

We come home and everything is normal. Or even better, really. 
I go into my room to clean a little, not sure how to act. Later she comes up to my room and says that what I saw, wasn't really what I saw. I am 201% sure of what I saw. 

I don't know if my brother even believes me or how to act.
<bot>: I saw my brother gf cheat on a guy and told him, now everything is acting normal and I have no idea what to do or how to feel.