How much should I charge in-laws for babysitting?

It would be 3 or 4 times a week, for 6 to 8 hours a day. The boy is 6 and the girl is 9, they are siblings and they are A HANDFUL! They are my brother in laws kids. They want me to babysit for free because I am babysitting them in my mother in laws house (the kids' grandma) where I live temporarily, but because they are so difficult I feel that I need to charge them, plus they only provide them food half of the time and usually it's not nutritious. How much should I ask per hour for both kids?