I ended up living with my brother's ex of 5 years while she was dating one of my best friends, and I had just lost my place to stay.
We ended up hitting it off, she broke up with my friend, and one night, I cheated on my boyfriend with her. I told both him and our girlfriend about it the next weekend, at the first chance I had to be alone with them.
So, after I broke up with them, her and I started talking about dating, agreed to be exclusive, and a few weeks after that started dating.
Her best friend/ our roommate says we need to tell my brother. I completely agree, but we've both been kind of stalling, because my brother has suicidal tendencies, and we worry about him.
He actually said she should hang out with me instead of him, because me and her are more alike anyway, so I feel like he might be OK with it, but I still don't know how to tell him.
By the way, nothing even came close to starting while they were actually together, in that time we met like twice.
Any advice?

'Our girlfriend?' You broke up with a man and she broke up with a best female friend of yours?
Did your brother break up with your new girlfriend or she with him?
How long ago? She was already with someone new when this story starts.
Talk about confusing.
I need a diagram.

You should have told your brother before, you started dating, and no you don't know that nothing happened, but then why do you care

'Our girlfriend?' You broke up with a man and she broke up with a best female friend of yours?
Did your brother break up with your new gf or she with him?
How long ago? She was already with someone new when this story starts.
Talk about confusing.
I need a diagram.

No, she was dating a guy. I was dating a different guy and a girl. I don't know when her and my brother broke up exactly, or who broke up with who. I don't talk to him very often. They were broken up, and not talking when the two of us started hanging out, and have only talked for a short while since then. Also, if you want confusing, I can give the full story with the 4 other people that were/are tangled into my three-way relationship situation, but, I'm out of that now so I don't see the point. I'm a guy, by the way. Also, clearly, I'm bi.

You should have told your brother before, you started dating, and no you don't know that nothing happened, but then why do you care

I was saying nothing happened between me and her. She lost her virginity to my brother.

Meanwhile, back at the mansion, go tell your brother all by yourself. Plan on spending at least an hour with him to see how he's doing, and save it for the middle of the good heart to heart. He's your brother!

Wow, what a mess. Really hard to keep track of what you're saying.

Let me give you an example of how we feel reading your post:

My brothers girlfriends roommate's sister slept with the neighbors brothers ex-wife's brother. Now I'm dating the ex. How do I tell my brother?

Bottom line, This is your brother's ex. He deserves to be told. He actually deserved to be told before you and the ex decided to start a relationship.