<human>: i don't know where else to go.

dating this woman for a year and things were going very well.  We're sexually compatible, we connect emotionally, and we share a sense of humor.  she seemed really into me but kind of distracted sometimes.  she texted a lot.  when i asked her about it she said "it's just my brother".

I put her on my cellphone plan and it turns out that she *only* ever talks to her brother, 10+ times a day.  Alwats nonsense like "I'm eating breakfast" or "I'm hitting the gym".  One time she said, "I'm about to shower then I'll head over"

I know it's her brother's number (we're all pretty close) for a fact.  They aren't actually related but have been together since they were kids.

This morning I went over to her apt and her car was outside but she wouldn't answer her phone.  i let myself in with my key and i immediately heard sex noises from down the hall.  cheating on me.  okay, i thought.  I can deal with this.

I opened her bedroom door because I wanted to confront her.  I don't know why.  it was a bad idea in retrospect.  

she was on the bed, her legs hanging off the edge, and her brother was in the room with her.  He was doing some shit that brothers should *not* do to their sisters.

I bailed out of there fast and started drinking.  I'm freaked out.  i don't think either of them saw me.  what the fuck do i do?

Can you give me the gist of the text in a nutshell?
<bot>: girlfriend in a an apparently consensual relationship with her "brother", saw them having sex, do I confront or just run?