Have you lost your mind? You have 3 kids, do you not remember how difficult life was after each of them was born? These are YOUR 3 children. It is not the place of your SIL or anyone else to take care of them. You chose to have them, now deal with it. How dare you be angry at her for not wanting to look after your kids. Plus it sounds like you are expecting her to do it for free too. Apologize to her immediately, send her flowers and tell her what a thoughtless jerk you have been and hope that she will willing to overlook your entitlement.