"My baby niece iris .. scroll through and whiteness [sic] the most squishy cute gumdrop ever!!" Bieber captioned the pic, above. "Love you @alaiabaldwin and @andrewaronow ! She's so precious!"
Johnson, who is a father of three, took to the comments section, writing, "This image pretty much seals the deal."
"Fully expect you and H to have a baby in 2021. 😉👊🏾," he added.
Baldwin, 23, also shared a photo with her sister's newborn baby, posting a pic on her Instagram Story.
"Auntie's girl," she wrote alongside the image.
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After Baldwin's older sister, Alaia, announced Iris' birth on Wednesday, Baldwin gushed over the family's latest addition.
"Been hard to keep this to myself, my beautiful niece is here and she is PERFECT," the model wrote on her Instagram Story. "Auntie Hails loves you sooooo much."
Both Bieber and Baldwin, who first said "I do"back in September 2018, have voiced their desire to become parents, but aren't planning on it anytime soon.
"I want to start my own family in due time," Bieber told Apple Music's Zane Lowe in February. "I want to enjoy being married for a little bit, go on tour, be married, enjoy traveling with just us, build more of our relationship. I think that it's definitely the next step, for sure."