All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible for you to work in building the house which you mentioned in the question, and the fact that half or all its expenses are from a Riba based loan does not harm you.
The sin of the loan with Riba (interest and usury) falls on the person who got the loan and not on you because you are not the one who borrowed the money and you did not help the borrower to borrow the money.
Therefore, you are permitted to work in building the house but you should advise your brother in-law about the prohibition of borrowing with Riba and that he must repent to Allaah from this.
For more benefit, please refer to Fatwa 81364.
Allaah Knows best.