I would say, "No, I have other plans and cannot help." You don't need to go into detail about what the other plans are. You may repeat yourself if you need to, but not too often. Don't get involved in long explanations. Don't get conned into trying to defend your decisions. If you're having a conversation over the phone and she starts belittling you, say, "I have to hang up now. Love you - 'bye!" and hang up.
This is not a one-fix solution. You may have to do this forever. It won't make you popular, but popularity or even love is not really involved here.
You can't change anyone except yourself! It's too bad this sort of thing happens in families, but it can be toxic, and you have your own family to watch out for. Only you can decide whether or not you will be manipulated by another person.
Be very careful how you talk about your mother and your sister to the rest of your family or to your friends. Don't let bitterness get hold of you! "Take the high road" in the matter; you want to model the right actions and attitudes for your son.
If at any time you want to help a family member, you decide what the help will be and how much. If that doesn't suit the family member, he or she will need to seek help elsewhere. And you still love them - 'bye!
You may feel you're having to harden your heart to save your sanity. But a manipulator plays on guilt, so you are probably not as hard as you may be afraid you are! You don't have to play the guilt game.