I know that everyone is saying to stay out of it, and I agree. But at the same time - you are curious/ or worried enough that this bothers you NOW. What about January/Feb when it could still go on? There's going to be a point in time where you're going to go crazy!
I agree with lending a sympathetic ear. And I think its good that you and your sister went to have coffee with her one day. Perhaps maybe plan another event like that. Does she like you and your sis? Maybe just stress some individual time with her and your bro. And maybe with her just kinda be like "omg i am so stressed about this and this...." and maybe she'll kinda open up a lil.
We all know that girls get a lil crazy sometimes.
So maybe this is just "i'm stressed and I need some me time" kinda of thing.
So while I would stay out of it and not be like "brother your wife could be a ho" *note - really don't say that*
Just lend an ear and let it be known to both parties that you care about them.