What should i do? problems with sister in law..?

Before you say or do anything..you really need to know if what this guy is saying all true...there could be another side to this story...can he prove any of what he is telling you, does he have these pictures? for the time being just sit tight do your job and just keep an eye on what is going on between them...it's possible he is angry with her because she would not get involved with him but it's also possible the is some truth in what he is saying...don't be in to much of a hurry to tell your brother ,until you know for sure she is up to her old tricks again.