Khloe Kardashian and Kylie Jenner are due to give birth "around the same time".
The 'Keeping Up With the Kardashians' stars are both reported to be expecting their first babies, and 20-year-old Kylie - who has been dating rapper Travis Scott for five months - is said to be thrilled her half-sister will be alongside her throughout every step in her pregnancy.
A source told People magazine: "Kylie is over the moon about her pregnancy. Khloé and Kylie are both due around the same time. They are approximately four months along.
"Kylie loves how close-knit her family is and is thrilled to bring that to her own family. She feels more prepared than ever, especially since she will be sharing this journey with Khloé.
"They are looking at it like a unifying experience for them as sisters."
The pair's mother and manager, Kris Jenner, is also there to support both 33-year-old Khloe - who is in a relationship with NBA player Tristan Thompson - and Kylie in their pregnancies.
The source added to People magazine: "Getting pregnant at the same time was totally unplanned but they are really happy about it.
"Kris is trying to be supportive of Kylie even though she's worried. But she particularly loves Travis and the entire family has known him for a very long time so they are more comfortable with the situation."
While Kylie's shock pregnancy is said to have sparked a mixed reaction from her loved ones, the family were much more delighted about Khloe - who was previously married to Lamar Odom and dated the likes of James Harden and French Montana - and Tristan's happy news.
A source said: "This is something Khloé has wanted for years and years and yet it's more than that: she didn't want it unless she was in the right relationship for it. She never let it happen with Lamar, intentionally, and she never considered it with James or French or anyone else she has seen.
"This is such a wonderful moment for her: not only is she going to be a mother, she is going to be raising a child with a man she truly loves. Everyone is over the moon for them both."