MIL sure has a tight rein on your husband and BIL. I'd be letting him know right quick that he best get to cooking cause it sure as heck wouldn't be me because I don't answer to his mom. If he does, that's his problem to handle.
It feels like you think you have to do this to get him to go along with a wedding. I can't figure out why else you are so desperate to participate in your own humiliation. The moment you wanted to practice it was your cue that you didn't trust him to do it.
Any time a parent pulls out the "if you live under my roof" spiel, you've already lost. You've set yourself up as an adversary instead of a resource. You're engaged in a power struggle with a 15-year-old just to prove you're in charge, and it's stupid and counterproductive.