I want to ask my step daughters to baby sit
Would you other kids be okay with her babysitting?
I am not sure. I think she would want to.
While it would be nice if she want to babysit, it is okay if she doesn't. She is not responsible for your children and you are. She might resent you if you force her.
It will be like they're going to school.
You shouldn't just expect that your step daughters would want to babysit for you. You need to make sure that they're okay with it first.