Would I be a partner in my brother's sin if he took an interest-bearing loan to complete construction, mortgaging the plot of land we share, knowing that I forbade him from doing that?

Borrowing with usury is forbidden by Islamic law and is considered a major sin. There is no difference between borrowing with usury for building a house or for any other purpose, unless a person reaches a state of dire necessity that permits what is otherwise forbidden. If your brother has not reached a state of dire necessity, then it is not permissible for you to agree to mortgage your share of the land so that he can obtain a usurious loan. This falls under cooperating in sin and aggression. Indeed, "The Messenger of Allah, may Allah's prayers and peace be upon him, cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses, and he said: 'They are all equal (in sin).'"