I posted before with this NC nickname about the problems my DB and SIL had with DM in the run-up to their wedding. They both spoke directly to DM by phone after earlier issues and she was quiet for several weeks (at least to any of us).
Then, on the day DM was a horror: trying to pick nasty arguments with everyone on our side of the family all day, aggressively finding fault with DB as he was stood nervously at the registrar's desk immediately before SIL was due to come in for the ceremony, alternating between comparing all of us negatively to SIL's family, and sulking because SIL's relatives didn't come over to talk to her. (I think they were probably terrified.)
When I left comparatively early I was publicly accused of a lack of affection for her because I didn't go around the table and kiss everyone like SIL's family members would do to each other. (I had a migraine headache and felt sick, had a small child half-asleep on my shoulder, and we're not a family who hug or kiss in public.)
After I'd gone apparently it all went downhill even more rapidly and ended with DB telling our mother that she'd ruined his wedding day and he hoped she was finally satisfied. There was apparently an almighty row which others had to calm.
DB and SIL had made things up with her sufficiently to attend another family gathering a few weeks later. After that they've kept to themselves for a few months and I don't blame them.
Fast forward to this weekend and DB phoned for a chat. After the general catch-up he raised the wedding but surprisingly he didn't seem that angry now. He asked me if I thought there was anything wrong with DM. He had tried to speak to her the morning after the wedding and had been disturbed at her apparently not remembering anything she said or did on his wedding day.
I thought he was implying dementia and I said something along the lines of "Oh no - she's always done this. Don't you remember when we were children and I had to write things down to prove later that they really happened and I wasn't going mad?".
(An example was when she burst into my room while I was trying on a swimming costume as a young teenager, ignoring me asking her to wait, and then screamed and raged at me at length while I stood there half naked trying to understand what she was so angry about. Apparently I said something wrong when I answered the door to a door-to-door saleswoman earlier that day and she had heard about it second hand. It may sound relatively minor but was scary and humiliating at the time. In DM's world this never happened and I was shaking and crying over nothing that day.)
DB said that this was precisely what he meant. Thinking about the irrational rages followed by apparent amnesia as a long-term pattern, he had come to the conclusion that there might be something seriously wrong with her mental health.
I said I wasn't sure and didn't think it would change very much at this stage. I silently wondered if DB wanted to believe that his mother was ill rather than just sporadically vicious to her own family for no reason.
I've been mulling it over since and still don't know whether she is just badly behaved or if she could have some kind of mental illness. Part of me thinks that even if DB is right she is still responsible for never seeking help. It wouldn't change anything.