Sounds like typical bullying to me, which probably means that HDM is right and your SIL has some serious issues.
Is she your husband’s sister or wife of brother? Sounds like she is the sister and the other SIL is married to her other brother but just checking.
Obvs if it is that way your husband will know if she’s prone to being a bit of a b**** (if you can ask him in a subtle way...). Does he support you when she says things or does he not notice?
Do you get on with the other SIL when the horrible one isn’t around?
How miserable that you have to go through this every year
I don’t imagine you’re the only one in this situation by a long shot though, so hopefully someone will offer some good advice instead of lots of questions like I have!
Best of luck x