Some really good points here. Snickers, I would bet folding money that the reason you're feeling so down right now is that the situation in your home for the last 2 months completely contradicts what Mama Tiger is telling you: SIL needs to go. I can't imagine how exhausting it must be to have to arrange your life to ensure that none of your children is ever left alone with their aunt! It's GOT to be fucking you up.
I really hope you can see that the only option is for SIL to leave.
It would be great if she got the hint that she's not really welcome here and packed up and left of her own accord, but I don't think that's going to happen. So that leaves telling her to sling her hook.
My own view (see upthread) is that you should sit her down and calmly and rationally tell her the truth, which is that her offer of help, while appreciated, isn't working out because since you witnessed the pinch, you can't actually trust her to be around the children unsupervised. Tell her the truth. You've been bottling it up for 2 months, and what's that got you? Untold stress, a terrible diet, a massive chocolate and takeaway bill and a SIL you still can't trust around the kids.
Look- she pinched your daughter's face in a manner that not one of the several hundred people who've posted on this thread could justify in a million years. She did that for one of two reasons. Either she genuinely believes that this is a reasonable way to discipline a year-old baby, or she is a dangerous, nasty piece of work who enjoys inflicting pain on those too small to defend themselves. If the former, you'll be doing her a favour by pointing out the error of her ways; if the latter, who gives an actual FUCK what she or any other member of the family thinks?
I also think you should issue marching orders before she leaves for Switzerland, so she knows she's coming back basically to pack up and get out.
Good luck with it- I really do genuinely know how hard it can be to deal with family. FWIW my own SIL is Australian and occasionally bizarre (not suggesting any causal link between the two
), but I blew up at her a few years ago- as in properly lost control of myself, shouting and screaming like a madwoman- and to be fair, it did the trick. That was after 10 years of treading on eggshells, so if I can do it, so can you!