Buying out sister's "Partner" ?

My sister has found herself in a difficult situation after buying a house with long term boyfriend. They purchased a house together last year but it is not working out for them and now he wants to split up.
He does not want to live in the house but she can't afford to buy his share.
She desperatly wants to hold on to the house as the thought of getting deposit for a new house nearly kills her.
I hate to see her in this situation and would like to help her out if i can.
So,
I am thinking of buying her ex partners share of the house and moving in to the house with her. we could hold on to the house for a fixed amount of time (5 yrs.) and then sell.

My questions are
1. What is the easiest way to go about this?
2. do i have to get out a mortgage with the same lenders that they have their mortgage with ?
3. Should we consider an "interest only" for the short term, considering we would sell in 5 years. (would this even be possible ?)

Many thanks for any help that anyone can offer

What are the sums like? As in, how much is house worth, how much is outstanding mortgage, will there be any stamp duty payable, what is your status as a purchaser (i.e. ftb, second time buyer, investor?), can you get a mortgage and if so for how much?

Ex partners share is presumably 50% of the equity in the house ( i.e. the capital value of the property less the mortgage) so new purchaser will either have to take over old mortgage ( may or may not be possible) and raise cash or both of you will remortgage to pay off old mortgage and raise cash.

Do the sums and post back.

mf

Sums are as follows

house now valued @ 290,00
Outstanding Mortg. @ 255,00

Equity is now @ 35,000

I am a 1st time buyer (so no stamp duty payable), i should be able to get 100% mortgage as my occupation is onw of those listed for 100% approval from some banks.
However i have no spare cash with which to buy out his 50% share of 35,000 (17,500) and would not like to take out seperate loan.
So if we went with a remortgage would i be able to borrow an extra 17,500 to pay this off or does it need to be financed seperately ?
Also i know i will be losing my F.t.b status but is there any other effects to look out for ?


Many thanks

If you do buy into the house with her, make sure you set up an agreement up-front about what happens when either of you want to get out of the property.