I definitely empathize with your situation but I think that your FI, sounds sort of silent on the issue and needs to step in and create some boundaries. Mothers are the pillars of the earth and while I am sure he dotes on her, he needs to reign that out of control behavior in YESTERDAY. It sounds like you guys are trying to build on your financial situation so you do not need her making unnecessary budget blowouts to you guys budget. If you have to pay for her, YOU handle the arrangements. Cancel the reservation she made and book it in a room tha is more in line with her budget. I learned early on that straight talk bring straight understanding so you need to have your FI's support and your FI sit her down and settle this. If he does not nip this in the bud now, it WILL get worse. Se already sounds like she needs prozac now,more so in a few months and years to come. Trust me, it will wear you down emotionally, I know!
Reset your budget together, talk to FI, get his support and let him handle his mother or let him know you will if he doesn't. But you cannot be confrontational about it. Good luck!