MerryMarigold in my humble opinion it is really no one else but the SIl's business who she tells about her past experiences, end of!
She has obviously chosen not to tell her husband and that is her choice. There is no way that the OP should reveal or even hint at anything of this nature unless it was a life and death situation, which it is not! That is my humble opinion. That sort of revelation, and possibly the brother's non sensitive replies and attitude might well break them up. What if he does not believe her, does not believe it was so bad, thinks it is an excuse et etc? He might say why have you not told me so far, why are you saying it now, is it all about our baby's name - and a way to control me into not choosing the name I want! (Sorry OP I am not saying he would say any of this but he might!)
We also have no evidence he would ensure he never pushed any buttons, he might, and he might do it accidentally, but with his wife's knowledge that he now has the knowledge of her abuse - she may well be less forgiving than if she had never told him.
Personally, I think honesty in a relationship is important and useful, but I have, very fortunately, not been abused so I have no idea how I would react or how I would protect myself from those memories and feelings!
I really hope those reading have not been through any such experiences, but even if others have and have chosen to act very differently and be open about stuff in a partnership, that doesn't mean it is right or even possible for all people to do so.
We are all different and the first rule for me in such situations would be not to make things worse, forcing revelations might make things worse, even if anyone felt they also might make things better! That is just my opinion and I hope it makes sense and does not sound judgemental, as that is not my intention. 