Babysitting

My brother girlfriend has two kids a 10 year old and 7 year old and she asked me to babysit this summer for her since I won’t be working this summer. I’m a lucky one who works during school season. I get summers off, my problem is I don’t know what to charge it be about anywhere from 8-10 hours a day 3-4 times a week. I don’t want to charge to much she’s practically family. Any advice would be great.