One thing that I was told when I started (from the person who helped me get going who had been doing it for over 10 years) was to NEVER EVER do care for family or friends. She ran into issues of her sister in law taking advantage of her and there became hurt feelings that to this day still cause tension at family gatherings. Just FYI.
I agree that all people must follow the same contract. If you want to hold a spot and not get paid that is fine but expect others to want the same treatment.
I had a mom on mat leave for a year. She still brought her older daughter 3 days a week (she was a minimum 3 days a week before). They lucked out and I had a spot open this past Sept for the infant. She didn't return to work until mid October but paid early starting sept 1st to secure the spot and ended up bringing both girls to transition easier.
I had another family leave in May. They had asked if I would take a lower rate to secure the spot and I told them no. The took their chances and gave up the spot. They have recently emailed asking me to let them know if anything opens up in April (now they will need 2 spots). Unfortunately it isn't looking like I can accommodate them. That was the chance they took in giving up their 1 spot.