Nicky Hilton Congratulates Sister Paris Hilton on Her Baby Boy: 'Welcome to Mommyhood'

Nicky Hilton Rothschild is congratulating her big sister on an exciting new milestone.

The fashion designer, 39, took to Instagram on Wednesday to congratulate Paris Hilton, 41, on the birth of her baby boy with a sweet, sentimental throwback of the two sisters as children, sitting on mom Kathy Hilton's lap.

"Welcome to mommyhood @ParisHilton! 👶🍼🧸 So so incredibly happy for youuu! 🥰," she began the caption to the photo, where Nicky wears a blue striped dress as she sits between her mom and sister, while Paris is in pink.

Nicky Hilton/instagram, MediaPunch/Bauer-Griffin/GC Images

"It's the most beautiful ride. There is no greater love. You are going to be the best mommy. What a lucky boy! 💙"

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.

RELATED: Paris Hilton Is a Mom! Star Reveals She and Husband Carter Reum Have Welcomed a Baby Boy

Also congratulating Paris on her growing family is sister-in-law Tessa Hilton — wife of Paris's little brother Barron Hilton — who shared the birth announcement on her Instagram Story and wrote, "We love you❣️Congratulations," tagging the couple.

Tessa and Barron's family has also grown this year, with the couple welcoming son Caspian in September, joining daughter Milou Alizée, 2½.

Paris and husband Carter Reum confirmed to PEOPLE on Tuesday that they welcomed their first baby together, a son, via surrogate.

"It's always been my dream to be a mother and I'm so happy that Carter and I found each other," the new mom told PEOPLE exclusively. "We are so excited to start our family together and our hearts are exploding with love for our baby boy."

Sharing the exciting news on Instagram, Paris posted a close-up photo of the baby gripping her thumb. "You are already loved beyond words 💙," she wrote.

Paris has gotten in some parenting practice with her sister's little ones, joining them on a Disneyland trip just last month to celebrate niece Theodora "Teddy" Marilyn's fifth birthday.

Christian Thompson/Shutterstock

Paris posed with Nicky and her girls — Teddy and oldest daughter Lily-Grace Victoria, 6 — as they met and took pictures with Disney princesses and rode the tea cups as a family.

Though Nicky's husband, James Rothschild, was pictured in a few shots, the youngest Rothschild, the couple's 6-month-old son whose name has yet to be shared, was absent from photos of the outing.