Jack Osbourne will be the father of a baby girl, his sister Kelly seemingly let slip.

The 26-year-old reality star is expecting his first child with fiance Lisa Stelly later this year.

But In Touch reports that the Fashion Police host inadvertently revealed the sex of the new addition to the Osbourne clan although she soon tried to cover it up.

"My brother already said, You're not allowed to spoil her!'" Kelly said but then hurriedly added, "Oh, I actually don't even know what it is yet, no one does."

Kelly had recently shared that she felt her younger brother had put his wild past behind him and was going to be the "best dad in the world."

"Not a lot of people know this about my brother, but this is what my brother has wanted for a really, really long time," she told E! Online.

Jack and Kelly are the children of Black Sabbath star Ozzy Osbourne and America's Got Talent judge Sharon Osbourne.