@justinsparky72,
I know this is an old post. No clue if your still checking this. I was in the same situation. However this is my wife best friend i was having an affair with.
Let me explain this to you a different way. Might help.
I had a full on affair for 9 months with my wife's best friend. We both are married with kids. She would invite me over to her house when her husband was away. I have sex in her and her husbands bed twice, in the back of her car, even at her husband in laws house guest bed. It's total disrespect bro.
I felt so bad. Like i just had sex in the bed her and her husband sleep in!? Like it does not get much more disrespectful then that. I never did the same. It was always at her house or in-laws.
a 5-6 year affair seems crazy long man. I had a 9 months full on (Sex, kissing, Emotional, Texting daily, etc) affair and on and off casual texting for a few years (Nothing crazy serious). You really need to find out what happened in the 5-6 years. Maybe sex was only a few times? Maybe it was emotional texting for awhile? You never know.
As for trusting her bro. Dude idk man. I would say no way.