Ouch.

Like it or not, you just made a very clear statement to your future MIL and SIL: You are not my family, but I still expect preferential treatment. Now, you probably (I hope) didn't mean to do that, but that is exactly how you're coming off. And to be honest, the fact that you are including your other sister in law but not your future SIL from the family your actually marrying into is pretty insulting.

I'm also seeing a distinct lack of empathy in your original post and all of your responses. Your question is less of "How can I help repair these hurt feelings and mend our relationship so we can be a happy family now and in the future?" and more of "How can I get them to stop whining because I'm going to have to see them at holidays, and oh yeah I want the brooch for my wedding?" No wonder your MIL is not being as responsive to you, instead of addressing the issue at hand you only going on about the damn brooch whenever you talk to her. She's not being petty, she's trying to process her feelings because you're acting entitled and it's probably coming off as a shock to her.

I don't think making SIL a bridesmaid is the answer here, because if you do it now they will know you're only doing it to try to shut them up. No, she should not have assumed she was going to be a bridesmaid, but I have this sneaky suspicion you let her down less than gently and add to the fact that she's probably really embarrassed now.

The only thing that is going to fix it is a sit down between the three of you, being honest about your feelings, and a sincere apology on your part.