The reality TV star, 34, and sister Kourtney, 36, both wedged themselves into racy black dresses that wouldn't have looked out of place in saucy film 50 Shades Of Grey.
Sharing the snap on social media, Kim - who is five months pregnant - wrote: "Banger sisters."
While recently-single Kourtney added: "Just a chill sister Saturday."
In the picture, the girls are wearing fringed wigs and purple gothic lipstick fit for Halloween (but a few months too early)!
Morticia Addams, eat your heart out!
Kim also celebrated reaching a whopping 42million Instagram followers with a racy shot of her cleavage.
The photo shows off Kim's chest, and only the lower half of her face is visible.
She also shared lots of selfies with her stunning family as they gathered to celebrate Kylie's 18th birthday.
One of the family snaps shows Kris Jenner and transgender Caitlyn proving that the exes are still on friendly terms after a difficult year.
Kim jokingly captioned the photo: "The parent trap."
Pictures of Kim with her four sisters Kourtney, Khloe, Kendall and Kylie were simply labelled: "Sister Squad."
A what a beautiful family they make.
Kylie kicked off her birthday celebrations at Nobu restaurant in Malibu.
The lucky teenager was given a new tanned leather Hermes handbag, even though her birthday isn't until 10 August.
Sadly, troubled brother Rob was missing from the celebrations.
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