Visit to aunt was very productive yesterday.
She now has a better understanding of how my mother is behaving. Whilst sad that mother and daughter can't have a reasonable relationship, she empathised when I put things into an historical context. In other words, this behaviour is nothing new. She listened, commented where appropriate, the most telling one being "I now realise it's not all you".
When touching on the problem with my sister-in-law, my aunt asked if I would have hit her back (if she'd connected with my face). My answer was no, I'd never behave like that. She agreed that sister-in-law has a problem which she won't acknowledge. The inability to "hold her drink".
The conversation was lengthy but at last I feel there's someone in the family who understands. My aunt even said she admired me for having my own home, car, etc. However, those are only material things. She agreed with the quotes I used from this site, especially about acknowledging my mother won't/can't change.