Whenever I hear something like this, I think of the movie Aladdin, when Jafar says, “Ever hear of the Golden Rule—whoever has the gold makes the rules?”Who is picking up the tab? If you are expecting your MIL-to-be to pay for an extravaganza, then you will need to come to a peaceable solution.
I think allowing the people who are paying to call the shots is reasonable, to a certain point. For example, if a request from the recipient is super outrageous, dangerous, or controversial, the giver can and should be allowed to say no. But in reasonable circumstances, I think there’s a real danger that the gift-giver can end up using money to unjustly control the gift-receiver. The OP’s situation seems like one of those circumstances.
When all is said and done, a gift with strings attached isn’t really a gift, IMHO.