I see this a lot here but also from another POV. Best friend for about 17/18 years but known her all my life as she was/is my childhood NDN and still lives 2 doors down from my DP's.
Her widowed DM is in her late 80's, very active still but has a heart issue and a severe hearing issue for which she has a hearing aid which she doesn't often use.
Friend is early 50s, youngest by 10 years to her older DB and DSis. Older DB has a wife with bi-polar disease and another medical issue plus 3 DC, 1 of whom is about to have her first baby and the other is a teenager still at school, third DC who I think is back at home. Her DSis still works, her DH retired last year and her 2 DS' are moved out and in jobs.
Friend returned to live with her DM after being made redundant/forced out of a badly paid job with accommodation provided (house) at 35, less than a year after her DF died after a short illness. Her DM has always wanted her to live with her and care for her and has said the same to her own friends (the DM's friends), which was partly why my friend returned home. She has talked about her DM selling the house, buying a warden assisted private flat (her DM's suggestion partly) and then she would get a property with the third share as rest would be split with siblings but this never happened, I think, partly this was before or after the stroke happened.
Friend suffered a stroke about 7 years ago and is back at work but being bullied, long story. She appreciates that her DB can't help out much but he does pop over when he can, he retired early but does other work. Her DSis on the other hand, rarely comes over and when she does it's just tea and cake/lunch visits.
My friend is now thinking about giving up work to be a full time carer for her DM, her DM doesn't want to employ private/NHS carers but the siblings have POA for their DM luckily.
This means though, that the 3 siblings have a third share in a house which is worth just over a million pounds now, probably more, as the last one was sold for a million in 2020. This house had had no work done to it since being sold previously (in past 2-3 years). But siblings are not paying care home fees, carers fees and my friend is looking to give up her career (not well paid, her DSis has a better paid job and her DB had a very well paid job).
This is quite a common scenario. Not quite the same as yours OP - your DB does sound like a manipulative character but really there's not a lot you can do. I'd move to be honest, otherwise you will be the one to care for your parents when they get old and infirm and your DB will get off scot free and do nothing. Be prepared to get threats that you miss out on your £90K inheritance if you do this though.