You do need to tell your sister as soon as you can. Have you even talked to her since you got home? She really needs to know what happened especially if she was thinking about a separation or divorce from her husband before this happened.
I know it isn't an easy thing to even contemplate doing right now and I know there are the fears that she will take his side. Somehow, though, I think this may be what she needs, too. I have been wondering how many of her friends or other women he has done this to.
I have said it before and I know you have said you don't feel it, but you are a very strong woman (even under the shock and confusion) and I think your children have a good role model in their mother.