What is the ruling on relatives and friends participating in a competition, knowing that some of them won, and a fatwa has been issued stating that this is a form of cheating? How can one expiate for this, given the inability to return the prizes?

If the website stipulates that the contestant must answer personally, then your participation using the names of your relatives is forbidden due to deception. The prize recipient must return it to the website, because it was obtained through an illegitimate means. You must advise them of this. However, if the website does not stipulate that the contestant must answer personally, then there is no harm.