Buying house jointly with elderly DM, how to arrange the finances

[insertinterestingusernamehere]

Hello, hoping someone can help as to how this can work, typing one handed as DS (14 weeks) is asleep on me and also a bit sleep deprived so please excuse grammar etc.
We are selling our family home and my mums in order to buy somewhere with an annexe for her to live in. We will need a contribution from her from her house sale to find the right type of property, but want to keep that to a minimum so will have a mortgage ourselves (and she will need to fund her many SAGA holidays ) I have one brother, who will eventually be 50% recipient of any remaining assets of hers (although most likely SAGA will have had it all by then ) I am not trying to avoid paying any appropriate taxes or care fees, but I am concerned about a possible future scenario where a third party is entitled to funds from the equity in our family home. We won't want to move again and eventually the annexe would possibly be useful for MIL to move in. Am I right to think that mums share in the house could be called upon to pay care fees?
God, this is getting long sorry!
What would be the implications if, for example, she gives us 150k cash and my brother 150k from the sale of her house and we use that towards the house? Instead of her actually owning a share in the house. For background, I get on well with my brother and he is not expecting this windfall, and my mum says "whatever you think best dear"

I thought there would be lots of mumsnetty help with this one...

I don't know the definitive legal position on this one but logically I'd assume that if your dm passed away you might have to sell to pay inheritance tax if she owned part of the property, I think there is a seven year period where there is a potentially exempt transfer (for example if she gave you money but you didn't have to pay tax) But I also think there is a limit of how many thousand (I think it might be £7k) your dm can give you each year?
Sorry not to be more helpful, maybe speaking to financial advisor or accountant would be a good plan?

Please don't take my comments as legally correct as it's just my point of view. Probably the easiest solution for you would be your Mother keeping in reserve a good amount for her 'Saga experiences' and then doing as you say halving the rest between you and your brother. This would work if you are expecting Mum to live another 7 years.
This would prevent the monies being used later if your Mum was to need to pay care home fees. There is a 'but' with this, as you or your brother, at a later stage, may feel that you need to contribute to your Mother's care especially in topping up the cost if you would like her to be somewhere decent if she needed care.
You would need to come to an arrangement with ongoing living costs with the house - maintenance and utilities.
In any event Mum would have to agree all this and it would be advisable to get legal advice

You need a lawyer for house ownership, and a way of protecting your home so not forced to sell for inheritance tax or care home fees or to pay your brother his share
Was her home originally your fathers too, did he use all his inheritance tax allowance if not three may be his share too taking the figure to £650k before any tax due

Care homes can not force a sale when part owned by another party

If she is unlikely to need care in next 7 years she could give you and your brother some money now

Your mother must make a will and signing powers of attorney to be activated lkater would also be a good idea

It sounds like she would be self funding for care home, can you have a right to buy her share later so it eventually is 100% yours