Baby on the Way for Evan Ross and Ashlee Simpson: 'They Always Wanted a Big Family,' Says Source

Evan and Ashlee Simpson Ross' little one on the way will join soon-to-be big sister Jagger Snow, 4½, and second-time big-brother-to-be Bronx Mowgli, 11

Ashlee Simpson Ross' Autobiography is about to add another family member!

The singer-songwriter and actress, 35, is pregnant with her and husband Evan Ross' second child together, the couple shared on Instagram Thursday alongside photos of themselves holding up a positive pregnancy test.

"Ashlee has been wanting another baby for a long time. They always wanted a big family!" a source tells PEOPLE.

"We are pregnant and we are so excited to share it with everyone. Baby #3," Simpson Ross captioned her post, while Ross wrote alongside his, "The fam is growing. ♥️ Ash and I can't wait to welcome the newest addition."

"During this unprecedented time, we know pregnant women may be under greater stress which is why we are supporting @marchofdimes Mom and Baby #COVID19 Intervention and Support Fund," the mom-to-be added, referencing the ongoing coronavirus pandemic. "Check out their Instagram page to see how they are helping moms and babies get the care they need now and in the future."

The little one on the way will join big sister Jagger Snow, 4½, plus the mom-to-be's 11-year-old son Bronx Mowgli, whom she shares with with ex-husband Pete Wentz.

The spouses celebrated three years of wedded bliss in August 2017, with the "Pieces of Me" singer posting a sweet tribute to their relationship on Instagram.

"Happy Anniversary my love 😍 it has been the best 3 years of my life ❤️," Simpson Ross captioned a throwback snap of the duo sharing a smooch on their wedding day.

"I love knowing that I get to wake up next to you for the rest of my life," she addressed Ross, 31, who is the son of music legend Diana Ross. "Thanks for giving me the best love."

It's impossible to tell whom the new baby will take after more at this point, but considering the very different personalities of Simpson Ross' two kids, it will definitely be a surprise.

"She's definitely different than Bronx," she told PEOPLE of Jagger in December 2016. "Bronx was an easy sleeper. She's like, 'Nuh uh, Mama. I want to stay up and stay up next to you!' "

But that "little bit of toughness" is something Simpson Ross cherishes deeply about her daughter. "I think it's good for a girl," she said of Jagger's outgoing nature. "I'm excited."