Did she explain "What breaks her heart?" (that you call because of obligation not love, that you didn't meet her expectations because you didn't make up with your sisters?) It's faulty thinking that she can get you to make up with your sisters. That she is trying, for whatever reason, only causes distress and makes it worse.

I'm not saying to never call but with all you are dealing with, I vote that you call even less often than once a week. Keep in mind that the toxic things she does only serves to make her more lonely (but you have to take care of yourself first--you can't fix her). FB posts to the world are not the same as true relationships. I am sorry your relationship with your mom is so traumatizing.