It depends on the situation. I paid my daughter to watch her brother if my husband and I went out for the evening, just as we would any other babysitter.
However, if I ran errands throughout the day and she was left alone with him with instructions to watch him (in essence - babysit), she wasn't paid for that. It was understood that part of being a family member meant contributing to what was needed for the ENTIRE family - and you don't get "paid" for doing what is normally expected of you to help around the house. (Frankly, if I got paid for cooking, cleaning, and laundry, I'd be a wealthy woman!)
That seemed to work for us.
Good luck!