Is it permissible to partner with one's brother in building a house and establishing a commercial project, knowing that he previously acquired money from which he earned bank interest, and that he does not fully pay the zakat on his wealth?

A person's participation in building a house is permissible. However, their participation in trade is disliked if they do not refrain from dealing with unlawful (haram) matters, such as one who does not avoid usury. It is obligatory for them to pay the zakat on their wealth and to dispose of any unlawful wealth that has become mixed with their lawful wealth, such as if they deposited it in an interest-based bank while knowing its impermissibility. They must avoid unlawful wealth because there is no blessing in it. Allah Almighty said: "And let not those who withhold what Allah has given them of His bounty think that it is good for them. Rather, it is bad for them. They will be encircled by what they withheld on the Day of Resurrection. And to Allah belongs the inheritance of the heavens and the earth. And Allah is Acquainted with what you do." (Al 'Imran: 180), and He said: "And those who hoard gold and silver and spend it not in the way of Allah – give them tidings of a painful punishment On the Day when it will be heated in the fire of Hell and scorched thereby will be their foreheads, their flanks, and their backs, [with the inscription]: 'This is what you hoarded for yourselves, so taste what you used to hoard.'" (At-Tawbah: 34-35).