I'm sorry your going through this. Remember that when you're going through a rut with a person EVERYTHING they do tends to annoy you and piss you off, many of those things annoy you more than they ever would on a normal day if you werent already at your witts end with them.
My advice to you, if possible, is to distance yourself. Be kind and respectful but keep your distance and don't let her become OVERLY involved in your life and your wedding planning. When she asks questions give genric answers or say you've already taken care of it.
You DONT want this to ruin your relationship with FI. If you think there is a chance at resovling things, try reading this article about what causes anger. Maybe if you can pinpoint exactly what shes doing to make you so upset, you can learn how to constructively avoid or deter those situations. Best of luck! Here is the article: It really helped me!!
Anger/Aggression