OP,
this is really quite likely to backfire on both your parent and the sibling that has taken on the house.
if your parent needs care in future, then if this had not happened they would understand normal circumstances have sold their house to pay for the care home.
if they need to go into a care home in future, and have no assets other than the house, the local authority (who are the people who pay if someone has no assets) can look back and see if someone has deliberately given away money or other assets that could have been used to pay. This is called “deprivation of assets”.
so most elderly people don’t give their homes away because otherwise they wind up in a situation where they need a care home, the state refuses to pay and they don’t legally own their home any more.
it’s also a problem with inheritance tax - again, the authorities are wise to this one and if a parent gifts their house to a child or other person but keeps living there then it’s treated for tax purposes as if the gift never happened.
https://www.ukpropertyaccountants.co.uk/gift-with-reservation-of-benefit/
so…. Your mum has given your sibling a poisoned chalice.
on her death the house will be treated as if she never gave it or part of it to your sibling/sibling’s partner and tax will be due.
I’m not a lawyer so I don’t know if it still counts as part of her estate and subject to her will or not.
either way I’d be running very quickly from this situation. I see what she is trying to do. It won’t work and it’s going to backfire messily.