If I were the sitter and my employer kept cancelling on short notice and not paying, I’d feel that I was being jerked around, taken advantage of, and disrespected. I’d start looking for more reliable employment.
You’re asking the sitter to keep certain days open for you. That means she can’t make other plans for those days. She may have expenses that require a certain amount of steady income. It’s not her problem that paying her is a sacrifice for you.
I’d be pretty upset if my employer could randomly cancel my work days on short notice and not pay me.
Maybe you could talk to the sitter about flexiblity and see how much is acceptable to her.