I think this weekend is the perfect opportunity for your FI to say something to his sister. I would encourage you to tell your FI to speak to FSIL about her behavior towards you. And it should all come from him, "Sister, I have seen the way you treat Knottie#s. I just need you to know it stops now. You don't have to be best friends, but you do have to be cordial. She is not going anywhere, so you better just get used to it."
Also, even though other family has seen the way FSIL treats you. FI should first speak to his sister privately. The last thing to help with situation is for FI to say something in front of everyone and they gang up on her. It could cause her to dig her heels in further in regards to you.
I don't have any advice for you, OP, but I can sympathize. This thread has reminded me that I need to go to bat more for my FW. Most of my family adores her, but SIL and brother can be really cold sometimes. My Mom's been advising me that "that's just the way SIL is and we need to be the bigger people," but I'm really over it. Hugs to you, OP.
Not to go off topic, but are ushers seriously not a thing? I envisioned it as a role where my brother could greet people and tell them to sit anywhere, nothing else. But I guess he could be a reader too.