firstly I apologize I was typing on my phone. Could you tell????
If you don’t want to ask the sitter ask the sister. She might be coming to you as a favor but she is not babysitting as a favor. She is babysitting to make money. So be honest with your friend and tell her you really appreciate that her sister is coming and you want her to be happy coming to you so please either find out or tell her not to be ashamed to tell you directly what she wants to get paid.
As far as chesed is concerned, it is chesed hours when kids go after school to help kolel couples who can’t afford help. But it is not chessed when kids babysit so you can go to the movies or to eat out with friends.