Not a good idea

You have to prepare to be stay in this marriage for 3->4 years after you marriaged to his brother. You have to explain this to your future boyfriend. You have to be responsible for him officially (including financially). Discuss these difficulty with your friend who ask for help, see what he would say. If you do want to help. I think he should at least offer 1/2 of the market price, which is at least 20k.