Jack Osbourne and fiancée expecting first child

To be a fly on the wall when Ozzy and Sharon Osbourne first heard the news!

The rock 'n' reality-TV stars' son Jack Osbourne is expecting his first child with fiancée Lisa Stelly. That means Sharon is finally going to be a grandmum and Ozzy will have five grandchildren!

Wait, what?

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Ozzy's son and daughter from his first marriage each have two children, but this is the first grandchild to stem from his and Sharon's brood.

"I'm excited. I'm a little nervous," Jack said upon revealing the happy news Tuesday during a sit-down with his parents for Piers Morgan Tonight.

She missed out on this interview, but sister Kelly Osbourne is obviously rooting for Jack and his soon-to-be missus.

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"Just spent the last hour on the phone with @MissKellyO ....she will be the best sister-in-law ever!" Stelly tweeted this afternoon.

Stelly's certainly joining a close-knit family.

GALLERY: Famous Families

"@shabooty shut your ... mouth!" Kelly tweeted in response to a hater who had written, "What an idiot, you're not supposed to knock up your model girlfriend :P."

Looks like Jack's little one already has the fiercest aunt in town.