Crazy future sister in law

I’m getting married in less than a 100 days and one of my fiancé’s sisters has been reeking havoc . She has always been the type of person who needs the attention on herself no matter if it’s good or bad yelling threats tantrums you name it she’s done it. Won’t go into everything she’s done over the course of our 10 month engagement so here’s her latest complaint my sister in law on my side my brothers wife she’s also my maid of honor was trying to nail down a date for my bachelorette party she sent me a text asking if this date would work for me I said it would but she called up my future mother in law and was angry because since she didn’t get to have a big 40th birthday in February when her birthday was because of COVID nothing was open especially big party venues so she had been planning to do a half birthday party celebration at my MILs house since she has a nice yard and a pool but never said what date she only said August she didn’t have a specific date in mind but as soon as my maid of honor sent out the date in the group chat with the rest of the girls who are going she said she was having her half birthday that day which was news to me and everyone in her family so now I’m worried this going to make her act worse then ever my fiancé doesn’t even want her at the wedding cause she always makes a scene how do I deal with this?