First let me say congragulations on your advancment!
Let follow up by saying, pardon me for being so blunt, whats actually the problem? You boiled it down this statment:
This means having to pay for daycare, and probably even a provider for her because we can't trust my brother in law.
The first part, yes. You would end up paying for day care if your MIL didnt live with you. I dont see that as a problem, per say, but yes an added expense. Given what your saving on driving (time, gas, wear and tear on the car) your making up for summer day care.
The second part i could not disagree more with you. YOU have made plans to care for her. YOU are perfectly willing to let her move with YOU while YOU continue to provide for her. If SHE chooses something different than that is indeed her choice. Not yours. And for god damn sure not YOUR problem to pay for some alternative.
See how that works? She made choices that lead to her living with you and being relient on you. YOUR house YOUR rules. She dosent get to dictate that somehow your wife has to meet a condition for you to continue to have the ”privilege” letting her live with you.
You need a little perspective, your MIL a needs a LOT