I'm going to charge my sister more when I babysit her kids than when I babysit my friend's kids
Do you think she will pay more?
Parents will always pay more for their kid's safety
Exploiting your sister by charging her more money just because you can is manipulative and downright wrong. If anything, most people charge family members less to babysit their kids. If you have financial constraints, tell her and I'm sure she'll understand, otherwise, charge the same rate per child.
But I can trick my sister into paying more
It is okay to want to make more money but you shouldn't exploit your sister for more money. It is expected to be nicer to your siblings since they will do favors for you too.