Erin Foster jokes about losing her inheritance because of dad David Foster's upcoming '$50M Wedding' to Katharine McPhee

Ever since record producer David Foster started dating Katharine McPhee, his daughters Erin and Sara Foster have trolled their relationship non-stop.

And now that the lovebirds are engaged and prepping for their upcoming wedding, David's daughters have only upped the ante when it comes to mocking the 35-year age gap between the pair.

This week, when a news outlet wrote a story titled, 'Rumor Has It: David Foster and Katharine McPhee Are Planning A $50M Wedding,' 36-year-old Erin couldn't help but screenshot the article and poke fun of the bogus headline on Instagram.

"There goes my inheritance," captioned the writer, who is two years older than 34-year-old Katharine, who found fame on American Idol in 2006. Older sister Sara, 38, then hilariously quipped, "Our inheritance was gone a long time ago FYI."

Famous friends of the sisters -- who both star in the VH1 reality show parody Barely Famous -- have also joined in on the narrative.

"I knew I should have dated him," commented US journalist Katie Couric, while Kate Hudson -- who is dating the sisters' stepbrother, Danny Fujikawa -- left laughing emojis on Erin's post.

Meanwhile, the playful trolling goes both ways because Katharine -- who got engaged to 69-year-old David last July after 14 months of dating -- has also dished out some nuggets.

When Erin asked why she wasn't invited on a trip to British Columbia that David took his new fianceé on, Katherine told her future stepdaughter, "Mommy and daddy needed some alone time.'"

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Single snap confirms star's age-gap romance