"why would she have had a relationship with her son up until he married and had children of his own?"
Smileless2012, It is not uncommon for AC to put up with things for themselves that they are not willing to subject their children or other loved ones to.
My sister-in-law's mother is frankly fairly terrible. She's not physically abusive, but she demands unreasonable things from her daughter. She also says awful things to her like "You know we (her parents) are the only ones who really love you. Everyone else will leave you eventually."
My sister-in-law told me she is unhappy with the relationship with her mother, but she doesn't want to lose her mother, so she bends over backwards to accommodate her, with no compromises from her mother at all.
Prior to becoming pregnant for the first time, SIL told me "If my mother doesn't change before I have children, I'm going to have to cut her off. I love her, but I will not accept for my children what I accept for myself."
My sister-in-law recently had her first child, and I see the end coming. She told me she did not want her mother in the room with her during birth. I mentioned it a couple weeks before and she got a look of absolute horror on her face and said "absolutely not". Well, her mother snuck into the delivery room and didn't leave when asked. She then posted a play-by-play of the birth on Facebook before my SIL even posted the birth announcement. In Grandma's own words: "I kinda just went in there to help...it was soon time...hopefully she felt better with me there and not resentful...I asked if I should leave. The father said yes, but she said she didn't care at that point so I took advantage of that"
So she stayed, despite being told she should leave, because my SIL was too distracted actively giving birth to tell her she needed to leave.
The cut off is coming. I can see it clearly.