AIBU to refuse becoming my mother-in-law's carer by proxy?

she will never speak to me again

That may not be a bad option

This is something she should be sorting out with her DB

What is her partner/DH doing to help?

This is outrageous on your SIL's part. This is between her and her brother. It is not her place to ask you to take care of her mother at the cost of your other responsibilities.

What you and DH decide is between the two of you.

As for SIL, let her cut you off. She sounds miserable.

Do not feel guilty. This is not your responsibility at all. Can you imagine one of your siblings treating your DH this way if the situation were reversed?

DH and I have talked a lot about how I can best support here, and he says the best way I can support is to take things off his plate at home (eg doing more jobs around the house, helping more with my step children when they would usually be with him/us) so that he can use that time to visit his mum and help out.

I work full time, my own parents are elderly and infirm, I have 2 step children, 2 of my own children, and a serious health condition which I have to manage carefully.

Just tell her what you told us here, and if she says it's not enough then your dh and his dsis will need to employ carers, there's little to be gained from you getting sick also.

Could be way off-base as an internet commenter, my take would be SIL hasn't liked you from the get go

if that is true, then the stress of the imminent death of her mum is giving her ample reason to project all of that onto you, with the added bonus of an emotional guilt trip to paint you as the unreasonable one... and by implication, she now walks on water in comparison to her "bad SIL"

Obviously your DH does need to split care of his Mum with his sister, but there’s no obligation for you to do so. And of course your children’s lives shouldn’t be put on the back burner whilst she’s unwell.

I would get your DH to tell her in no uncertain terms that car of their Mum is between the two of them and doesn’t involve you.

They can remember that paid care can form part of the overall package if your DH can’t manage half of the work required.

Your husband's selfish to let his sister do most of it but she's just as bad for alighting on the nearest woman (you) and her expectations of you, which are likely due to her own misogny/socialisation. Possibly also MIL is putting pressure on her rather than him but that's not your problem either.

I would push back very hard to him - (if willing) you'll take some of the load off him wrt your own household for a limited time on condition he takes some off his sister AND tells her she's wrong to target you. You simply block her/don't engage.

As (some) others say, no adult has the right to expect free care from anyone. IME as a woman you sometimes have to push back extremely hard on this. I had several arguments with a social worker and some nurses (all women) who criticised me and/or pressurised me to look after my demented mother whilst I was also doing a stressful 60 hour a week job and lived 25 miles away from her. No such pressure was put on my brother, because of course, he never took her to medical appointments or went to meetings about her and because it's much easier to bully the woman sat in front of you.

Given your AIBU is "Am I really responsible?" it suggests you would benefit from implementing stronger boundaries and dropping all this nonsense about keeping the peace. That's just used to keep women from asserting themselves.

She is talking about expecting me to do things like taking MIL to appointments, cleaning her house, organising personal care, buying and preparing food for her.
Does your MIL even want you to do all of these things? If you have only been married 9 months I'm assuming that she might not know you that well? My experience of elderly people is that they want to hold on to their dignity.

Is there money available to pay somebody else to do house cleaning and food preparation?

If your MIL needs personal care, she really needs carers.

perhaps with only a few months left to live.

She may therefore qualify for an NHS funded end of life care package.

https://www.england.nhs.uk/personalisedcare/personal-health-budgets/personal-health-budgets-in-end-of-life-care/

I was in a very similar situation with DH's sisters when FIL's health was failing. We held a line that I wasn't getting involved and they should speak to DH. I took on extra things at home to release DH to help with a very overdue move from a stupidly large house to a much more manageable flat. He also did all the probate work when his father died. I deliberately stayed off the family WhatsApp group and handed the phone to DH if they called.

His sisters no longer speak to me which is a blessed relief. Now their mother's health is deteriorating and we are holding the same line. They speak to DH and I do not get involved. Once MIL is gone I hope to never see either of them again because the level of entitlement has been off the scale.

Tell her that you have family duties of your own. You don't expect her to help with your elderly parents, your children, your housework, and you have a full time job on top. If you could help more, you would. As it is, the solution you and Dh have come up with is that you carry the burden of all family and household stuff to free him up to spend more time with his mum.

If SiL is being used by the family - ie if she is ending up taking lots of unpaid time off work to attend hospital visits etc, you need to discuss this with her and come to a fair arrangement. Her anger probably stems from being run ragged.

Of course YANBU, this is NOT on you to have anything to do with.

This is on your husband to sort out, and fast.
How convenient your marriage is.
He can dump his children on you.

As for your new SIL, let her not speak to you again.
Absolutely no loss.

I can only think this marriage is a huge mistake.
Your position of skivvy aupair and now carer.🙄.
You also have your own health issues?

Tell your husband to sort his family out quick, or you will be re thinking the marriage.

I would be hugely concerned you are being used.
Your own children should be your priority, especially since you have health issues.

What exactly was the benefit to you of marrying him?

WHAT!!
OP does not need to do as much for her MIL as her parents anymore than her husband needs to do the same for his ILs as his MIL.

I am the carer for DH with dementia. Caring is very hard work and I understand that families can become very fractious when care is discussed. I know it is distasteful to say so, but it looks like the care needs will be short lived. I would consider bringing in some resource to supplement what you can reasonably do yourselves. on the advice of my LA I used the UKCIL website which allows you to contract with carers directly. It might also be useful to contact the Carers Matter charity, or at least look at their website which has some really useful tips.

Your SIL sounds quite angry. Understandable if her mum is dying, so try not to react too much. However, your life cannot go on hold and really there is no need for it to. Some disruption will come your way, but you really need a good and practical discussion across the family on what you can do and what you cannot.

It absolutely will not be in your MIL's interests if you set yourselves up to fail. If you find the right carer who can support it will feel much more like a friend has popped round rather than a carer.

Also worth touching base with Social Services for a needs assessment.

It's tough, I agree you shouldnt be expected to do more/cleaning /personal care in these circumstances, I suspect you are getting the worst of your SILs overwhelming anger, grief, guilt and stress as a venting mechanism. While this might be understandable if she's struggling, it's not fair or helpful.
I don't know how I would approach it but I would try and meet with her( together with your DH) and discuss. The strategy would be to listen, ask her how she's doing, smother her with understanding and kindness, but keep your boundaries.
Explain your paragraph about all your responsibilities and what you are doing to free your DH up to help already. Ask her who she thinks will look after your elderly parents and kids if you don't have time to, or become unwell? Gently suggest that the whole world cannot come to a halt because her mum is needing care, and that the kids need stability and safety, not the drama of distressed adults and cancellation of normality, which is likely to make a traumatic and sad situation worse, such overwhelm is not helpful.
Suggest that some things could be done differently eg food delivery, cleaners, her and your DH can look into availabile assistance and care for their mother so they can concentrate on the big things like appointments and spending time together with her, rather than tidying the garden.
Of course a lot of this might be driven by your Mil making impossible demands on SIL, or the usual 'find the nearest female' attitude from them both.

Really hope things improve for you all

It might be worth exploring whether she is eligible for continuing NHS care if she is terminally ill. In any case she needs to claim Attendance Allowance which will pay for a few hours of care or domestic help.

She is probably just totally overwhelmed with it all. So if the family can't cope then outside help needs to be brought in.

She will never speak to you again? You can but hope. I'd hold her to that.

She’s very angry and sending me messages saying she will never speak to me again if I don’t “do my bit”
Reply, 'bring it on/make my day punk' then block & ignore her

YANBU - she's not your mother, having ovaries does not make you the carer for your partner's family. Making more time for him is a reasonable contribution, but you can't be expected to step in.

You could tell her that's it's amazing she feels that way and you will so look forward to her hands-on help with your parents when the time comes.

Then back your phone up, drop it in the loo, get a new one and forget to tell her the number.

You need to be really firm.

“SIL I am being really clear here, I am really sad that MIL isn’t well but I simply cannot do this for her. As you know I work full time, have children and my own elderly parents. It is not appropriate for you to keep asking me to take over MIL’s care and I am not going to be able to take it on. I will ask DH to speak to you about working through a solution but I think this is something you need to sort out the two of you.”

Separately, your DH needs to discuss a long term solution with her. Has she got carers? Attendance allowance? Can she go into a home, even for a short period. He needs to work out a solution with his sister.

Sorry you are going through this. Its fair enough for her to expect your dh and sil to split your mil's care 50/50, hopefully the majority isn't falling on her. Its fair for you to take on the extra at your home to enable him to do the 50/50. Its very unreasonable of her to expect you to run about.

Supporting your spouse at home to free up time is a normal level of support. It’s not healthy for any of you to burn yourselves to the ground. Hired help exists and MILs resources should be used for this purpose at end of life.