My sister (in another state from me and my mom) manages all my 92 yr old dad's (he is mid-stage Alzheimer's) outside medical care , has him visit her home, does all his personal matters and engages him in activities,7/days wk--he is in skilled nursing facility but she is there to engage him for hours each day. We need to formalize relationship,payment, duties, expectations, taxes, etc.
I think you are very wise to be formalizing the care arrangement.