Nowhere is it written that just because someone sends you a message or calls you, that you must reply or answer. That's one of the great things about caller ID.
Respond when you feel you are able. If they ask, you were busy when they messaged you. Learning to separate yourself from the crazy is a difficult thing to do. Try to not respond until you feel like you are separate from their crazy.
If you need help separating from their crazy and not getting sucked in, imagine them to be someone you were chatting with in a grocery store. Respond to them in the same way that you would talk to someone you don't know and will never see again - that is, be polite and civil. You'll have nothing to hold against yourself if they behave in unacceptable ways. You will have been reasonable, even in the face of their unreasonableness.
But this is just the first step. Learning how to separate from the crazy you've known all your life, and STAY separated is not trivial or easy. Some people like ACA meetings for learning some of this, others prefer a therapist. I wish you well in your progress towards controlling your own life and not allowing others to control you.