What is the Sharia ruling on what my brother did by forging contracts for an apartment with an unknown owner, selling it, and disposing of its price? And is the apartment considered the right of all residents of the building? And what is the best course of action towards my brother if he does not respond to returning the money, and will I be held accountable for his actions if I leave him alone?

Your brother has committed a grave sin by encroaching upon the apartment, whether it is owned by another person or their heirs, or if it is common property among the building's owners. You must advise him by explaining the gravity of his action, that it is a form of consuming people's wealth unlawfully, and that he should take from its rent what he spent to complete and furnish the apartment, and return the remainder to its rightful owner, or he should take what he spent from its rightful owner. He must reclaim the apartment from the one who sold it to him and give him back the price he paid for it, unless the rightful owner of the apartment approves the sale, in which case the price would be theirs. If he does not respond, escalate the matter to the person in charge of the owners' association.