Well, groundhog, that's what you should tell her. You love her and the kids but won't/can't be there 24/7 as you need your time too. My DH left for Spain on Thursday for a work/holiday jaunt for god knows how long. He was obviously feeling guilty about leaving me recovering from major surgery as he started arguements for no reason. I told him that it was HIS decision to go and that he does what he wants. I didn't tell him I won't be there when he gets back as I've made my mind up. It just takes something to make you snap. When you have your hysterectomy you most certainly won't be able to do much for your sis, so maybe it's time she returned the favours.
Oh and I have 3 sons, one spoilt rotten by his grandmother and the other 2 are great cooks/iron own clothes etc as they were brought up by me! Their partners are very grateful!
ps; Trey, maybe I could do a hysterectomy advice section because the medical profession don't tell you much!