What is the ruling of Islamic law regarding the marriage of a young man imprisoned on charges of theft and forgery to his paternal uncle's daughter, while concealing the matter of his imprisonment from the girl's family?

It is not permissible for you to propose to your relative on behalf of your brother, who is imprisoned on charges of theft and forgery, unless you inform her and her family of the truth of the matter. Concealing this and claiming that he is traveling constitutes deception and falsehood, which is contrary to the advice due to a Muslim. The Prophet, peace and blessings be upon him, said: "Religion is advice," and "Whoever deceives is not of me." Deceit, treachery, and betrayal lead to Hellfire. The jurists have agreed that deception is forbidden, both in word and deed, and this includes concealing defects. If a person sees a defect in the suitor or the one being sought for marriage, they must disclose it as advice. Such a matter will likely be discovered later, causing problems. Your brother must repent, and you must inform the girl and her family of the truth so they can decide whether to proceed with the engagement or dissolve it. If it is difficult for you to reveal the matter, you can dissolve the engagement using any excuse or stratagem, and by employing double entendre (tawriyah).