Joanna Gaines is an aunt again!

The pregnant Fixer Upper star, 39, announced her sister Mary Kay McCall and brother-in-law David welcomed their sixth child with a sweet family photo montage on Instagram Thursday.

“My sister had her beautiful baby girl! #motherofsix #butshelooks16,” Gaines, 39, captioned the photos.

The baby news comes two days after Joanna and husband Chip Gaines, 43, announced they are expecting their fifth child.

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My sister had her beautiful baby girl! #motherofsix #butshelooks16 😍

A post shared by Joanna Stevens Gaines (@joannagaines) on

RELATED GALLERY: Tour Pregnant Joanna Gaines’ ‘All Time Favorite’ Fixer Upper, Designed for Her Sister

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I was laughing this morning when I started getting texts and messages congratulating me on my new baby! No, I didn't have a baby and no, I'm not pregnant 😉but my sister shines and I'm so proud of her! #numberfive #notme #her

A post shared by Joanna Stevens Gaines (@joannagaines) on

In another photo, the mother-to-be posed with Mary Kay (who they call Mikey), older sister Teresa Criswell and their mom Nan Stevens.

Joanna previously celebrated the birth of Mary Kay’s fifth child on Instagram in February 2015.

Fans of Fixer Upper have already been introduced to Mary Kay. On Tuesday night’s episode of the hit HGTV show, Chip and Joanna took on their “all-time favorite” project, making over a home in Waco, Texas, for her younger sister.

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Today's house was my all time favorite #fixerupper ...we got to reveal this house to my sweet sis and her family of seven (going on 8!). I can't contain the excitement-after nine years they are finally coming back to Waco! Can't wait for you to see this episode 😍 #vintagemodern #plantladiesforever

A post shared by Joanna Stevens Gaines (@joannagaines) on

The same night Mary Kay’s episode aired, the Gaineses surprised fans with the news that Joanna is pregnant.

“Gaines party of 7.. (If you’re still confused.. WE ARE PREGNANT) @joannagaines,” Chip wrote along with a photo of the couple cradling his fake baby bump while Joanna shows off hers under a grey sweater.

The couple is already parents to children, Drake, 12, Ella, 11, Duke, 9, and Emmie Kay, 7.

Fixer Upper airs Tuesdays at 9:00 p.m. EST on HGTV.

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