Yes, obviously she is taking advantage of me as the on-call babysitter and that's even more proof she isn't thinking right.
So why did you agree to it?
It's not fair to him in the least, and I'll agree it's not the best thing for his mental and emotional well being, but... you can put up with pretty much anything for your kids.
If I only saw my kid two days a week, I'd be the on call baby sitter, too. Every day is precious, and something you can never, ever get back.