First off please do not worry about the money....my mother is on social security as well and I understand the difficulty's with todays costs. I am not here for the money but to help you find a balance that you and your daughter can live with. This is what I do and what I love
I am sorry to say that your son is right.....there is nothing you can do to force her to change. Though the good news is there are things you can do to not allow her to treat you as her personal punching bag. (More in a mental then physical sense). The sad reality is all to often our kids just assume that we are there for them to treat as they wish and that our feelings and needs are second to their own. While we as parents just want the natural love and attention that comes from having children. Sometimes when one side is pulling or pushing more then the other this becomes a battle field.
The truth is someday she will see what she has done and come to regret her choices. But you can not allow her to treat you disrespectfully XXXXX XXXXX day comes either. I would not suggest a confrontation as this will only fuel her fire. When a issue pops up that needs addressing then do so. Let her know at first sentence that you do not want to argue, or be screamed at. Let her know you love her and this is the issue that needs addressing. If she begins lashing out.......calmly explain to her again that you love her but will not talk to her until she can speak with you like a adult. Then hang up the phone or walk away.
Now I know right now your thinking, no way! If I do that she is going to be madder then fire and I will not see the grandchildren. The fact is she is already holding the grandkids hostage regardless of what you do when she gets angry. The real thing is she needs you to babysit and help out so she can only be angry for so long.
Make sure during these periods that if the grandchildren wish to come over you still allow it as they should not suffer for moms actions. Send birthday cards and letters to the grandchildren or consider calling and asking to speak with them. If she refuses stick with email or letters. Make sure you shoot your daughter a letter as well to let her know you are thinking of her and miss her. Make sure you always let her know that you love her and if she wants to talk that you are more then happy to. Just explain that her behavior toward you has caused you pain and you can not allow it to continue.
If she calls again.........talk to her. If she blows up, again calmly explain that you will talk to her when she is calm and can treat you with respect. Then hang up or walk away.
This may take a few months or longer to get under control. But in the end she will either have to treat you with respect or not speak with you. I know you are looking for a way to turn everything into a perfectly happy family, but based on what you have told me there is no quick fix or anything you can do to change her. She must choose to change herself........and the only thing you can do to help her is to stop enabling her. Every time you allow her to treat you this way it fuels her mentality that it is OK. When in fact it is not ok.........you have a right to be happy and not be treated like this.
Sadly if she does not learn to curb her aggression she will run her children off. The sad fact is few people wish to be around a negative person and she is making herself toxic to everyone around her. By standing up and being firm you are teaching her a great lesson on how people deserve to be treated. As parents we never stop teaching........the hard part is some of the lessons we must teach can be painful at best. But if you allow her to continue then she has learned nothing on how to respect someone. In the grand schema of things treating your mother with respect is the norm......this is the up-most job of a child and if the child does not treat mom with respect then it is unlikely she respects anyone. Which will make her life harder in the end.
Again I wish I had a magic wand I could use to help........but she must make the changes and you must stop enabling her destructive behavior. By doing so you may teach her a lesson as well as show your grandchildren that everyone deserves to be treated well.