Prince William and Kate Middleton joined the rest of the royal family for their annual Christmas walk in Sandringham on Christmas day, and were accompanied by Prince William's step siblings, Laura Lopes (aka his "secret sister") and Tom Parker Bowles.
According to Hello! Magazine, Tom and Laura attended the Christmas day service at St Mary Magdalene Church, but "made a private arrival." Meaning they didn't show up in photos of the royals like this one:
FYI, William reportedly has a somewhat tense relationship with his step-siblings. In fact, the Daily Beast previously reported that he was considering skipping Christmas if Tom showed up due to Wills not loving his—as a source put it—“rather louche lifestyle as a Mayfair bon vivant."
Meanwhile, William has also reportedly had a dramatic relationship with Laura over the years. Royal reporter Katie Nicholl said in her book Harry and William (via The Sun) that “William and Laura used to have terrible fights over who was to blame for their broken homes. William would blame Camilla for all the hurt she had caused his mother, which would send Laura into a rage. Laura was not having any of it. She would take a hard line and fire back at William, ‘Your father has ruined my life.’”
Seems like this drama is in the past, though, considering the royals just spent the holidays together and no major issues has been reported (um...yet).