I would look on your calendar and find at least 2 dates that you need a sitter and ask her to watch your kids on either date. If she says no to both dates and doesn't offer to watch them another day, you can say something like it would be great to trade off babysitting, maybe another date and you need to get back to her with another date. If no dates will work for her for the next 4 months and she doesn't ever offer to babysit, then you know she is just using you as a sitter.
If you want to continue to babysit to see the kids, I'd be very selective about how often you babysit. You decide how often you are willing to do it (once a month or 2 times a month or every other month). Stick to it.
When you agree to babysit, tell her an exact time to pick up the kids. If she doesn't pick them up at the agreed time, then the next time she asks to babysit, say yes if she can pick them up at the designated time. There has to be consequences. If you set the boundries, she will agree or you don't babysit. And you can say it nicely. Like, I need you to pick up the kids between 10-10:30am. If she asks why, say I have other things I need to do that day. If she questions you, then you can say, Sorry. I really can't babysit if you can't pick them up between 10 and 10:30am. If she balks at the terms then you can always give her the name of your babysitter, who she'd have to pay if she cannot be more respectful of your wishes. You are doing her a favor, so favors mean that they should be reciprocated or at the very least be respectful of your time. You can always say no.
Practice on your husband before telling her. If your husband is willing, he can set the boundries with her and tell her these things since it is his sister. But don't avoid making the change. It just gets worse with time. Good luck!