I read that earlier today. I took note of the fact that the only reason we now know this about Mark is that he admitted it. These are things he did a long time ago, and which he now admits are wrong. It’s possible for people to grow and change. I hope he has done so, and it appears he has.
However, cheating on an exam is not in the same league as pretending to the world he had a brother he never had. He’s owned up to his wrongdoing in the cheating case. For a permanent lie/deception there is no similar confession. If there were actual evidence that he’s lying about David: i.e.: if a person had been located who knew the family back in the late sixties, seventies and/or eighties, and who claimed Mark never had such a brother, then it would be Mark’s word [and A LOT of other people’s word] against that lone voice. If more than one person were found, it would certainly begin to look as if a deception had occurred [even the Bible says a lone accuser is not enough; two or three witnesses is required]. So far there has been no one, not a single, solitary person stepping forward to dispute David’s life and death.