Harry's mother Anne married Robin in June 2013, although he had been part of their life since Harry was young.
Harry walked his mother down the aisle, and was even Robin's best man for the occasion.
Friends and family have taken to social media to share their condolences.
"So sorry to hear your sad news xxx," one person wrote.
"Oh no, so sorry for your loss xxx."
Robin's ex-wife and daughter have both changed their Facebook profile pictures to feature him, in tribute.
Harry has yet to make any statement about the news yet, but fans are urging people to let the family grieve in peace until they are ready to make any public announcements.
Rest in peace Robin Twist <3.