house build loan only for 6 months

Doglover88

bit of a strange one this so please bere with me,
my sister is looking to build a house on some land the family own, and has been quoted approx £150k for the total project. If she was to borrow all the money to complete the build, then sell her existing place and use that money to pay off the loan for the build after say 6 months, what would she be expected to pay back in total?
I guess an 'early redemption penalty' wouldnt be written into a loan that was expected to be paid off early, but i assume they'd be wanting more than £150k back?
The reason I'm asking is I'm considering buying my sisters existing property and suggested paying her for the house so she can use my money for the build (and obviously doing me a good price on her place ), therefore not needing a huge loan (and the interest)
Many thanks in advance