A gift rom a sibling comes under the class II threshold for
gift tax,so you pay tax at 20% on all gifts/inheritances
over that threshold(45000 aaprox).
You will pay stamp duty also on the site.
The least beneficial result for you is that you are deemed to take a gift of the completed house at market value as then you will pay stamp duty on the MV.
It would be important to structure the transaction to avoid this. If you get a loan latter written off or a gift of cash from your brother there is no stamp duty cost. If your brother engages builders to build your house ,then you will have SD on this gift.
It would be worth getting good tax advice on this as different ways of carrying out the transaction will have very different tax costs.
Your brother should also be looking to best structure this transaction to his benefit. The least beneficial scenario for him is that he is hit for CGT on the disposal of the houses to you,the best scenario would be if the transaction was regarded as part of his trade of building.