Kourtney Kardashian is planning to celebrate her 21st birthday again this week.

Kourtney Kardashian

Kourtney Kardashian

The 39-year-old star has promised her youngest half-sister, Kylie Jenner, that she'll join in with the celebrations at the lip kit guru's party and claim they are both turning the same age, because she pulled a similar stunt when their other sibling, Kendall Jenner, now 22, also reached the milestone.

Discussing the plans for Kylie's birthday on Friday (10.08.18), Kourtney told E! News: "I think we're gonna do a big party...21!

"She wants me to [act] like this is my 21st birthday also. I think I did that for Kendall's 21st. I was like, 'Guys, it's my 21st birthday!' Like I pretended like it was my birthday too. So Kylie was like you better be pretending like this is your 21st also. So it's a lot of pressure but I'm going to have to have a lot of fun that night."

Kourtney - who has three children, Mason, eight, Penelope, six, and three-year-old Reign with former partner Scott Disick - thinks it is "special" to see how Kylie and her other sister, Khloe Kardashian, have adjusted to parenthood.

And the 'Keeping Up With the Kardashians' star thinks they have become closer now that Kylie and her partner Travis Scott are parents to six-month-old Stormi and Khloe has four-month old True with Tristan Thompson.

She said: "It's amazing watching them... and I think I can also now say, 'now you can see how I did it, why I was feeling this way.' I think we just relate to each other and I think you never know how you're going to be as a mom, so watching them and how they're doing it - like everyone has their own way. You really can't predict what you're going to be like.

"So I think just watching them is really, it's really special to see their journey.

"I don't even know how we got here with so many babies but it's so amazing. Just the fact that our kids can all grow up together...it's such a magical time."