The 18-year-old took to the social media site to share a shot of the siblings posing for a portrait in a wood panelled room with his 12,000 Instagram followers.
Dylan appeared to be dressed in a formal Armed Forces uniform, carrying a sword as he stood by his sister, 15, who was seated on a chair in front of him.
“Just a young guy looking for princesses,” he captioned, adding: “@theroyalfamily hook me up!
“Or take my sister, at least I would get to go to a royal wedding,” he joked.
Fans took to the comments section to praise the glamorous snap, with one saying: “You already are a prince and princess.”
“After this pic you two should get a random invitation for all the royal weddings,” another insisted, while a third added: “What a picture!”
Meanwhile, the pair’s famous mother Catherine, 48, also had her say, quipping: “When the cat’s away the mice will play!
“Love you both so much.”
It seemed the pair had been playing dress-up while Catherine was away, with Carys dressed in a black lace dress with a luxurious white fur jacket over the top.
She accessorised with a glittering tiara as she and her brother took on royal-like roles for the day.
Catherine and Michael, 73, welcomed their first child together, Dylan, just months before they tied the knot in November 2000.
Their relationship has seen them through ups and downs in the their personal lives after the Basic Instinct actor was diagnosed with throat cancer in 2010.
A year later, he confirmed he was cancer-free, three months before his wife checked herself into a mental health facility to help treat her bipolar disorder.
She later told InStyle: “I’m not the kind of person who likes to shout out my personal issues from the rooftops. But with my bipolar becoming public, I hope fellow sufferers will know it’s completely controllable.
“I hope I can help remove any stigma attached to it.”
The couple briefly separated in 2013 but later reconciled, with Michael telling AARP Magazine: “I don’t think there’s much chance of fixing a relationship if one of you is already out the door.
“It took work on both our parts.”
Catherine added: “I think we’re both mellower and wiser. That comes naturally with time. We count our blessings.”
Michael is also father to son Cameron, 39, from his first marriage to Diandra Luker.
He became a grandfather for the first time last year when Cameron welcomed his daughter Lua Izzy with girlfriend Viviane Thibes, 40.
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