Upsetting info, but need to make a decision
So my ex (of almost 3 years) broke up with me about 5 weeks ago. There weren't very substantial reasons at the time as to why she did it, other than saying she was "unhappy". We recently met for the first time and things went well but said we shouldn't jump back into a relationship and need to at least date other people for a while. I agreed to this, because I want things to work for us, but also because I realize jumping back into something too soon without figuring yourself out is not a good things.
More background... HER FAMILY, our mutual friends friends, and of course my family support me overwhelmingly and are really upset at her for what she has done. Usually a breakup of an amazing relationship like we had is due to some sort of cheating or violence or something really bad. Nothing like that has happened between us, which of course makes me even more perplexed as to why she broke up with me.
Deep down I knew there was a possibility that there might be someone else, but I just never wanted to face it. Although a couple days ago I found some SUBSTANTIAL evidence that supports her going behind my back and being with someone else within the last couple of months... possibly leading to her breakup with me. She thinks I have no idea about this and basically thinks she can keep me on the backburner until she tries out this new fling (IF I am right with this theory, of course).
Anyway her brother is in town and really wants to meet with me and talk because he's pissed at what she did to me. I want to let him know about this new info I know about. I don't want to directly accuse her of anything... but I am almost certain she is lying and being manipulative and going behind my back. I won't stand for being played. If her family finds out, they will certainly give her a reality check (which is what she needs). I don't want to hurt her, but I can't keep this info bottled up to myself. Should I explain my concerns and suspicions to her brother when we meet up? I won't accuse her but simply discuss with him what I see and hear.