Am I sinful for what my maternal uncle did by bribing the police officer, given my knowledge of it and my failure to prevent him?

The prohibited bribery is that by which a person attains falsehood, or what is not rightfully his, or a right that could have been obtained without it. As for the bribe by which a person attains his right, which he cannot obtain except through it, or by which he repels an oppressor's injustice, its sin is on the one who takes it, and there is no blame in paying it, provided that there is no other way to repel the injustice.

If the questioner believes that his uncle's action is forbidden and remained silent without denouncing it despite being able to, then he must repent and seek forgiveness, because denouncing by the heart is not sufficient except when one is unable to denounce by hand or tongue.