G.B.
I agree that when she starts her "I'm so tired but I'll do it" just say "Oh dear, I wasn't thinking, I will find someone else. You work way too hard to have to watch these guys." Then hang up. Do it even when you don't have plans so if you really can't find someone else you aren't stuck without a babysitter.
This will teach her a couple of things. It will teach her that you respect her enough to allow her to not work for you. It will also let her know you are listening to her and hearing her when she says she is tired.....
Have some fun and start cultivating some new baby sitters. Only see this woman when hubby is around. No other times at all.