Hi eiregal
The ownership of the house and the name on the loan can be separated.
So your mother can buy the house in her sole name, but the loan can be given to you and to her jointly.
As you won't own the house, your first time buyer status should not be affected. You might want to confirm this directly with Revenue or via the solicitor.
I would guess that if you still have 100k outstanding in three years, when you want to buy your own home, your borrowing capacity will be reduced by 100k.
If the three siblings get on well, I would keep it as simple as possible and use only one person to go as joint borrower. I would probably go for the person who is furthest away from buying their own home.
At a later stage, you can change the name on the mortgage to one of the other siblings. At that stage, perhaps one of the children will have built up sufficient equity in their own home to remortgage and repay the mortgage on your mother's house.
But I would focus on solving the immediate problem of keeping your mum's home now.
Brendan