So much of your thread resonates with me. I was brought up by a (covert) narcissist "mother" who has made my life hell, especially in recent years as she has aged. I had to step back from her, felt huge guilt "abandoning" a woman in her late 80s. But she pushed me further than I could cope with. 2 1/2 years NC now and loads of therapy. I hope you might be in a position to get good therapy yourself, I think you need it to absolve yourself from your misplaced guilt. Also read about Fear, Obligation and Guilt, commonly known as FOG.
Obviously it's a terrible situation and you have to feel for someone who has MS. But they don't have the right to demand you set yourself on fire to keep them warm. With a newborn and 2 other children, you have every right, in fact you have a responsiblity, to look after your relationship with them. Your mother being as damaged as she is is not your fault, nor your responsibility. She never dealt with her issues.
I've a few comments on some of your posts, you may find some of them a bit brutal, however they are kindly meant -
"I'm just quite angry but she acts like this is all what anyone would do for a parent and also like it's not really that much but I'm exhausted and barely see my DH."
This is common with narcissistic parents. They are entitled and don't see you as a separate human beings with needs of your own.
"I've always had a difficult relationship with her. ... But I do love her and what her to be in a safe situation."
Not surprised about the difficult relationship, I had a sense of that before getting far into your 2nd paragraph. As to love, I think you're probably trauma bonded to her. Do you think you would love an elderly neighbour if they expected that from you?
"The guilt is horrendous because I know she won't look after herself. And I know she genuinely believes I should do it."
This is like some horrendous game of chicken which she's intent on "winning". She doesn't see the cost to you, or even to herself.
"She could afford a carer.. altho her rent is quite high. She just doesn't want one she says."
A question I learned, far too late in the day, is to ask "What about me?".
You actually matter, but I suspect, like me, you were brought up to feel that you didn't, that her needs mattered above everyone else. I would also point out that your father enabled this, which has caused a lot of damage to your emotional wellbeing.
"The thing is she doesn't have much interest at all in being a mum or a gran."
Exactly like mine. My kids are adults now, she never babysat or minded them, just criticised them and my parenting. My biggest regret is that I didn't walk away from her decades ago. But as a narcissist she felt hugely entitled even though she invested nothing in my children. The end of our relationship was because although she never gave them her time or interest, she felt that during covid they should have stepped up and looked after her. She told me if I was a proper mother she would have a better relationship with her grandchildren. I know it's not likely your mother will see your children into adulthood, but want to point out that if she has no interest now, that is not going to change. I wish someone had alerted me to this when my kids were little.
"I don't think I could live with myself if I didn't know I'd at least tried to do the right thing by her."
But you have. You have bent over backwards to meet her demands, potentially damaging the relationship with your husband to do this. No loving, caring mother would expect this of their child.
"I know the things she does are bad
.. bit I can't stop feeling so sorry for her and so sad. She has no one. And alot of that is to do with her yes.. but it's still deeply sad. I don't want her to die. I want her to be happy. But she is very hard to make happy.."
Thing is, people have to live with the consequences of their own behaviour. Sadly your father enabled so much (probably damaged from his own childhood). Her having no one is on her, and it's not your responsibility. I know it's really hard, but truly it's not your responsibility. You may not want her to die, and want her to be happy, but this is something outside of your control. You cannot make her happy (trust me, been there). You need to learn to let go - which is why I think therapy would benefit you.
"And my children love her.. and they were so excited she was coming back but that makes me so sad too because she doesn't really care that much about them.. particularly my daughter. She massively favours my son and that breaks my heart (tho I shelter my daughter from it and she hasn't noticed.. that will be harder as she gets older)"
And you think you love her?! How would you feel about anyone else who favoured one of your children over another? Honestly, this is something you must not allow. The damage that will be done to your daughter (who will have noticed despite your best efforts) is huge. Your mother is not going to change so the only thing to do is keep the children away from her.
"She has this way of making me feel abusive. She reacts very badly to anything that could be perceived as criticism so you end up avoiding saying anything that could be seen that way. Have to spend massive amounts of energy thinking of the right way to phrase things so that she might listen. She doesn't tend to listen unless it's what she wants to hear.
She constantly tells me and has always told me that I'm oversensitive and too anxious. I feel that I haven't properly explained that I find it too much... "
Typical narc. She'll make you feel like shit but don't dare say anything that she can (and will) twist to make it seem you're criticising her. You are wrecking your head to say things in a kind way, but she keeps moving the goalposts. No, she will only listen to what she wants to hear - she sees you as an extension of herself without any autonomy, therefore you must agree with her at all times.
Being told your oversensitive etc. is something else narcs do, I had it all my life - but you know, strange thing, it only worked one way. I could repeat the same thing back to her and she'd lose the plot, how dare I etc. And trust me, it's not that you haven't properly explained - it's that SHE HAS NO INTENTION OF UNDERSTANDING.
"She didn't speak to me throughout the entire pregnancy including the birth.. only after she ended up in hospital and needed to come back to the UK did she actually answer a call from me. She didn't once ask about the baby or the birth."
It's quite clear that you only matter insofar as what you can do for her. She has no interest in you or your life. And yet you think you love her. It is a hard thing to realise that you don't love your mother, because what sort of awful person would you be if you didn't love her. But it's not love, it's trauma bonding. Love isn't conditional, love is on both sides. The relationship between you and your mother is not love.
"I can't dictate it without her permission as we jointly own the property. (Something she is in denial about and won't discuss)"
Can you get legal advice?
I know you've more posts but I reckon the above is quite a bit to chew on. I sympathise hugely as I'm still having a very difficult time even though I haven't seen my "mother" in 2 1/2 years. It's horrible and neither you nor I deserved to have mothers like this. But she's not going to change, so if you want things to be different, you have to change. I'm so sorry.