Bless you @MorningSunlight . You have absolutely no obligation to help her when she has gone out of her way to crush your spirit and make your life miserable.
Be free, block her forever if you need to, and feel absolutely no guilt (easier said than done, I know. I'm sorry). Alternatively, just limit the contact to interactions to help and support your SD, under your control and on terms you are happy with. You are an adult now, no longer the 7 year old, cowering in the corner from your mother's rage and spite (apologies - I make this allusion because it's what happens to me every time my mum starts on at me - I instantly revert to the little girl in the corner, frightened, confused and wondering how I can scramble to make it better and make her happy).
Honestly, with such people you would be doing the best thing for you and your loved ones by dropping the rope and not letting them drag you under. They are the architects of their own isolation. Cut them right off, block on all media and continue to enjoy your freedom. You owe them literally nothing. But I can understand why you might not be comfortable doing this and why you might want to continue to be there for your SD.
You could contact their local social services department and request an adult care needs assessment for your SD. They might be able to put in some support for him/your M, but you don't have to pick up the reins of care and intolerable burden for someone who has been abusive. It is right that there is some obligation, indeed honour and privilege, in supporting someone in old age or ill health who has been there and done the same for you when you were young and vulnerable. But if someone has perpetuated little else but misery and abuse during your younger life then I firmly believe that that relieves you of the obligation to step up and be their personal nurse/chambermaid/whipping-boy when they become less capable. You reap what you sow - maybe now is the time you can leave her to reap the harvest of the miserable existence she bestowed on you?
You might find a couple of the long-running threads on MN helpful to read. There is the "Stately Homes" thread here February 2025 Well we took you to Stately Homes | Mumsnet so-called because the originator's parents were similar to yours but denied that their childhood was abusive because they'd taken them to National Trust properties.
The other recommendation is for the Cockroach Cafe - Cockroach Café 🪳 🪳 🪳New Year 2025 | Mumsnet a space where posters can get advice and support for dealing with elderly, often difficult and/or abusive, relatives. It is full of kind, understanding people who have been through it themselves and can offer wise support, practical solutions and empathy. They might also be able to help with suggestions as to how to get some more support and care in for your M and SD.
And pay no heed to those (on here or elsewhere) who pronounce themselves horrified that you could "ever speak that way about your parents". "You only get one mum" is a favourite standard, as is "I could never do this to my family, they gave birth to you!". Drown those voices out. Those people are lucky enough to have had normal, loving, healthy family relationships. They cannot understand the damage done by parents such as yours and will never get it, because happily for them they never had to. Their judgement and criticism of you is valueless.
And please don't blame yourself. This is NOT your fault. You sound like a lovely, strong person despite, not because of, your mother. I'm sorry that you had such a mediocre person pretending to parent you. It's not fair. It's OK to be angry and to grieve for the childhood you never had. But it wasn't your fault that they treated you like this. Block and don't look back, if that's what you want/need to do.
This is another helpful resource for people trying to do their best in similar family situations: Out of the FOG | Personality Disorders, Narcissism, NPD, BPD. In this case, "FOG" stands for Fear, Obligation, Guilt. The sorts of things you feel when trying to pull free from toxic family like this.
Wishing you all the best for the future. x