Rumer Willis. Picture: Reuters

Rumer Willis has joked that Shawn Mendes could be her new stepdad.

The 'Empire' star couldn't resist poking fun at her 56-year-old mother, Demi Moore - whose ex-husband, Ashton Kutcher, was 15 years her junior - when the 'Ghost' actress shared a photo of herself with the 20-year-old star at the Met Gala earlier this week.

Demi captioned her Instagram post: "My tablemate. Love this kid, such an artist and a sweet soul!"

Rumer - whose dad is another of Demi's ex-husbands, Bruce Willis - commented: "My new stepdad."

Her younger sister, Tallulah Willis, added: "A++"

Ahead of Demi's post, Rumer, 30, Tallulah, 25, and their 27-year-old sister Scout all used their own Instagram accounts to heap praise on their mother as she walked the pink carpet on fashion's biggest night on Monday.

They all shared pictures of Demi at the event and added their own captions.

Rumer posted: "No words....I know I may be a little bias but in my book my momma @moore2d wins the #metball game (sic)"

Scout wrote: "SHE'S F*** ING KILLING IT ELEGANT ANGELIC SUBDUED CAMP ALL THAT HAIR IS HERS (sic)"

And Tallulah quipped: "mid 50s mother of three heads for a quick run to the corner store for pop chips and flavored seltzer, stumbles upon this pink carpet thing. annoying but ok (sic)"

Even Bruce's wife, Emma Heming Willis, was impressed.

She commented on Tallulah's post: "Hot damn!!! [heart emojis] (sic)"

Meanwhile, Rumer has previously admitted it was "weird" when her mother got together with Ashton - who she split from in 2011 after six years of marriage - because she'd had a crush on the 'That 70s Show' actor when she was a teenager.

She said: "It was definitely weird for a minute but I have to commend him. He was a really great stepfather. The perspective shifted very quickly."