She might want to read up on toxic families, it's certainly a form of abuse and having started when she was so young will obviously have had a profound impact on her. She sounds very well adjusted and strong and she's lucky to have your friendship and support.
Overly involving your children in your messy adult relationships can be a form of control, roping the children in to the drama and making them feel responsible for the happiness and wellbeing of their Mum, Dad or both.
Completely different scenario, but my XHs DM was a great one for calling with incessant dramas and problems 4 or 5 times a day most of the time - not to say she didn't have them, she had loads of them - but refusing to listen to advice or make any changes leaving him desperate on and after every single call that she kept on and on in the same vicious cycle of awfulness (same response to each different drama) and he couldn't do anything about it.
He started by saying he wouldn't answer calls after a certain time (partly to give us both some time off from it), then that he'd only take a certain number of calls a day, and it did make a difference. His guilt about not being to help her, despite her not actually wanting any help, was the worst thing and when parents do a number on you like that you can end up feeling obliged to answer every call in case this time something really has gone wrong. But if you try a new way of managing contact you ave to stick to it to give it a chance.
Different tactics worked at different times but when things were relatively settled so he knew she just wanted to offload, again, him calling her at the same time each week gave her something to look forward to and she tried to save it all up and he got to prepare himself.
Sounds like it could be worth a go. "Mum, I know you're going through a hard time at the moment and I'm sorry. As you know, we're in the middle of loads of changes, living out of a suitcase, work very busy etc, and I want to be able to concentrate on our conversations which I can't do as things stand. I'll call you at 6pm on Tuesday and we'll catch up then. I won't be able to talk to you about this beforehand as I'm shattered and have too much on. Love SIL".