Help sister-in-law or not?

My husband and I have been married for nearly 20 years. We’re in our 50’s and we have both work hard all our adult lives. We have no children. His sister is also in her 50’s. She has been married and divorced 3 times and has 5 children. Two of the kids are at home with her. One is unable to live alone. The other is just starting high school. Sister-in-law is very needy. While she appears to be able bodied, she makes it known that she is in pain, fatigued, etc., and has never worked outside the home. She was in a car accident some years ago which has affected her neck – to what extent, I don’t know – but it is a point of focus for her. My widowed father-in-law has taken care of his daughter most of her life, and gives her money and buys her things (appliances, car). He is now elderly and very ill. While he will likely leave his daughter money, he seems to be trying to lay the groundwork in order that we basically take up where he left off. After receiving an email from his dad, my husband seems to be caving in to the guilt and pressure placed on him by his dad to “help out”. I feel extremely uncomfortable with this arrangement and feel I should have some say in this. I believe his father respects me and I
did voice my opinion about the situation. I feel that she is a grown woman, capable, and of well above-average intelligence, who can learn to do for and provide for herself. I am not sure he heard (or wants to hear) the message clearly. How do my husband and I, diplomatically, tell his father and sister that we do not want to carry on the family tradition of taking care of her?