does this sister-in-law have a husband? a broad hint that you are going to tell him would snap her head around, i bet.

otherwise, you have the right to tell her that she should never, ever touch you again. that if she does, she will live to regret it.

i have very strong boundaries and it isn't right for anyone to try and get past them. no standing within 18 inches of my person and no touching, without knowing that it is okay with me.

i would keep this incident between you and the SIL and warn her that if it happens again, you will do something about it.

good luck, pat