At least you know now...

This could be a double FML... By both of your siblings! =O

that is f'ed up from start to finish...dump the *****...beat the crap out of your brother...and break your sister's phone for good measure...

You better thank your little sister.

I don't see why the sister should get out of this unless she did it to intentionally bring it to light. If she didn't know, she intended to **** up what was thought to be op's good relationship. If the girlfriend wasn't cheating it could've made their relationship. Both siblings are terrible(unless she meant for him to find out about it).

Kick your brothers ass, or set up your brother and sister and make them both look like creeps. Ruin love for then forever

I tend to try and advocate a peaceful resolution to problems, within a household, but I would beat the shit out of Steve.

Best case scenario - he won't confirm or deny because he thinks you should know better. And she only said it to hurt you like she thinks you hurt her. I've been in that position; I told my ex husband, after he admitted cheating, that our son was actually his brother's child. That was completely untrue; we'd never fooled around. But I wanted him to feel my pain. I hope this is the case in your situation, OP, and that you can all sort it out.

Well I guess it's good you broke up then!

Wow. Kinda glad your sister texted her. That's terrible though. I'm sorry.