Is there a limit to what is considered reimbursement when a parent lives with you?

Maybe you could draw up a "lease" that states room and board for $750 per month. Have your Mom sign this lease [if she is able to understand what she is signing]. You might want to do an estimate on the cost, such as $300 for the room, $250 for groceries, $50 for mileage, etc. and attach it to the lease.

For myself, I have co-signing ability on my Dad's checking account, thus I can pay any bills he has. But the bill for caregivers comes out of my own checking account because it was always an effort to get Dad to maintain a good balance in his own checking, and I didn't want those checks to bounce. Thus whenever my Dad did finally transfer more funds into checking, I would write myself a check for "reimbursement". I keep a 3-ring binder with a running tab with copies of the bills.