Create a caregiving contract that very specifically lists what your duties will be, how much you will charge, when you will be available. Don't forget taxes, social security, workman's comp, etc. It's best to (at a minimum) have an elderly attorney review the contract and let you know what is valid in your state.
If you're looking for a funding source with which she could pay you check out this link - https://www.agingcare.com/articles/how-to-get-paid-for-being-a-caregiver-135476.htm