I think you already know what you must do. You must stop giving your sons money, and you must protect your parents from being exploited by their grandchildren. What it sounds to me that you need is the authority and self-confidence to refuse to be used by them, and the willingness to risk being abandoned by them (while knowing that such "abandonments" usually are temporary, however hurtful they are in the immediate moment).
I don't have any quick, simple answers for gaining the authority, self-confidence, and self-respect you need to stop tolerating your sons' exploitation and elder abuse. But I do recommend that you seek counseling from a qualified mental health professional. The Pastoral Solutions Institute
may be able to either offer telephone counseling or a referral to a counseling provider in your area.