My Father (88) has primary progressive aphasia (form of Dementia). Any advice?

He fell and was in the hospital. My husband and I have taken care of my alzheimer's mother and father for 3 years 7 years ago, now mother has been in assisted living. Father has been living with sister. I was unpaid when they lived with us. Now, this is my profession, they have asked me to watch him from 1:00 - 5:00 because they know that my earlier shift is from 6:00 12;30. I do not have a problem with it at all, my question is this, what would be the harm in asking them, (my sister is the one that is taking care of his money at this time), to pay me. As I overheard them talking about hiring an agency. Also, I asked my father friday, when I was watching him, Dad, do you want me to watch you or is that not what you want, his comment was, I want you to make money. To me that is an odd comment, my parents fortunately have money. So, of course they know that I am going to watch my own father unpaid, it just seems courteous because they know I am looking for a paid position for those hours, to pay me a little something.