I've considered that too. My brother might very well be also going outside of the M, as our father turned out to be an unrepentant philanderer. If that's the case then perhaps he's trying to spare our mother seeing him take a similar route. I don't really know.
But I somehow doubt it. Something tells me that if he had in any way been unfaithful prior to the separation, my SIL would not have pulled any punches in letting us know about it -- she certainly played every other card at her disposal.
At worst, I am certain that my brother is "looking", and even anticipating the day he is actually D'ed now. Part of the reason I think he's greasing the skids for his D is because his W hasn't allowed any sex with her for going on three years now. He's been as loving and as patient as a saint with her, but I think he's long reached the end of his rope. I am certain of that.
During Thanksgiving week he and his kids visited with my mom and our younger brother, B. One evening, he went out with B to a bar in a restaurant, taking his teenage daughter along. He was openly looking at the females, entertaining picking one of them up. In the end, he didn't, but my brother B was disgusted with him for such behavior right in front of his D17. (FYI, B himself is far, far from being prudish. Quite the opposite. But as a child of D who took the brunt of it in our own parents' split, B knows firsthand the pain these parents can cause a child.)
I just don't think they really understand what message he and his W are delivering to D17 and her siblings.
