OH. MY. GOODNESS. She is your FSIL? So you’re marrying her brother? What a nightmare. I’m really sorry.
You should fire her as Maid/Matron of Honor. You should. You should tell her directly that you know she doesn’t like any of your plans and that you don’t want to force her to participate in them. That’s what you should do, totally. If you don’t want to do that, here’s my other advice:
The only thing I will say in her favor (but not her defense because her attitude is unacceptable) is that if you want to choose the location of your bachelorette party, you should at least offer to pay your own way. However, maybe your other ladies already offered to pay. If this is the case, I would enlist another bridesmaid to run interference on this issue if you can. Have her contact your Maid/Matron of Honor and casually ask about the bachelorette and this and that. If the Maid/Matron of Honor pushes back, she can give her the option to opt out of the party. something like “cburger really wants to do this, but if it’s not in your budget, don’t feel pressured to participate.”
As for the other issues, if you aren’t going to fire her as Maid/Matron of Honor (do this) you need to reign her in. Is she always saying things behind your back, or does she say them to your face? who are these people who are telling you about it? Do they stand up to her? Do they tell her to chill the F out? They should. You might need to consider asking them to stop telling you about her weird freak-outs behind your back. When they tell you these things, they are piling on unnecessary stress and it’s not productive or helpful. You don’t need to know. You need to be zen.
If she is saying things to your face, agressively or passive-agressively, you need to take her on. I would be direct and say something like “I’m trying to be excited about wedding planning, Maid/Matron of Honor. You can give constructive suggestions, but please don’t be rude.” Or, if you want to take a lighter tone, you could try somehting like “You’re raining on my parade Maid/Matron of Honor, I’m trying to get excited here,” and then just let the conversation unfold naturally. If it’s all going on behind your back, you should consider confronting her anyway. Assuming you still don’t want to fire her (you should really consider this excellent option) you can politely “give her an out” so to speak, and also let her know you know what she is saying. Just like the firing suggestion above, tell her you know she doesn’t like your plans and you don’t want to force her to participate, but instead of ending that with “so go away” or “so I think it would be better if you attended the wedding as a guest,” you end that sentence with giving her the option to bail, something like “are you sure you still want to be involved in the wedding? if you want to just come as a guest, you can.”
But, in case my thoughts aren’t clear, I think you should fire her.