Basics such as housing, utilities and food are traceable/easily figured. Food planning, drug administration, cleaning, cooking, transportation, etc. aren't quite as straight forward. There's also the time involved for phone calls to manage health care appointments and issues, as well as bill paying and budget monitoring. I maintain two separate budgets for myself and my adult dependant. However, I seem to end up shorter and shorter for my own finances.
As always, if you want legal or accounting advice you can rely on, consult an appropriate professional.
Are you basing this on running two households. If Mom cannot make it in her home, then time to downsize or place her. If she is living with you, I agree she should be helping with housing, utilities and food but the rest of your post comes with having her live with you. Comes with the territory lets say. I did it all and never even thought of being paid for it. Mom didn't have all that much so being paid would have taken money I could have used on her so it really made no sense. I did reimburse myself for any out of pocket I had concerning her personally. Like Depends. I may charge them to my acct but reimbursed myself.
I think you need to speak to an elder lawyer to see what rights u have. SS is has rules to how a persons SS is spent and it must be on them.