Must a man who guaranteed his relatives an investment in a trade that incurred losses and their money was lost, return their money, knowing that his own money was also lost in the same trade, and he owns properties that he can sell to repay them?

The contract between the man and his relatives is a corrupt Mudarabah (definition) (profit-sharing) contract because it guarantees them profit, and consequently, the relatives bear the damage that occurred to the money. The hand of the Mudarib (investor/manager) is a hand of trust, and the ruling for corrupt contracts is the same as for valid ones regarding liability. So, if the money is damaged without transgression or negligence on the part of the Mudarib, there is no liability on him, whether the Mudarabah was valid or corrupt.