Princess Crescent Sparkle is the distant cousin of Princess Luna and Celestia, they don't feel closely related to her cause either she lives far away and is making her own galaxies and moons (for a reason) or they don't talk much any more or have never made contact since who knows.
Reason for not having contacted the princess Luna and Celestia:Edit
Well, it is sorta complicated (or i haven't thought of it) 1000 year's ago Luna, Celestia and Crescent where planning to make ponyville, Celestia was older then Crescent but she was older then Luna, together they made a plan to create the world you know as Equestria, Celestia said, "Since Luna is a bit too young and has not earned her cutie mark yet... why don't you show her an example of how to make a moon. I'd love to. Crescent said.. she took Luna and they both walked to a hill, the sky was a bit empty she said to Luna, "Um first i gotta make the stars.. i think i'll make picture's in the sky that remind me of Star Swirl the Bearded my old teacher... perhaps a new generation of pony might be interested in learning about him then i was, hehe." she did just that but made many more, then the moon... "Oh she said.. you gotta have moon raising power's just like Celestia has sun raising powers.. perfect but in an opposite way.'' she then made the moon a perfect crescent, the stars sparkled above making it look like a face, "I did the same thing once before little one.. it was an accident i wasn't planning on it to look that way.. but after that my cutie mark came and i knew making star's in my own way plus making a glowing forum at night was for me indeed!' Luna was confused, she tilted her head in confusion.. "Oh.. alright little one" she used her magic to put the moon in a perfect spot for Luna to practice. "There... now little one i want you to turn around so your back faces the moon and like Celestia when you fly raise both hoove's and keep on flying till it's high enough for all too see..,' Luna was almost there but failed. "Aww it's ok little one.' as Luna was about to cry, Celestia came down looking sad, '"Well i have discovered.. me and Luna are the only one's that can keep balance here.. i'm afraid your stars where all we needed and you teaching Luna a little..." "What?.. what... oh i see, it's just about you and Luna is it?.." Celestia said, "No... look i know Star Swirl knew your destiny.. but i don't think he said here... you ran out excitedly before he told you... that you had the power to create your own place in another place." Crescent looked sad.. but not crying.. she sighed, "OK... i see you know better then i do that he told you that and... and... fine... i'll leave.. but i'll be back and prove that i can do this on my own... good bye.. cousin.." she flew high into space... and just as fast.. winked out.." "Celestia then said... Oh.. Cresent Sparkle.. i love you young cousin... but your destiny isn't here. Cresent Sparkle was never mentioned again... did the princess's forget?... No.. they didn't:
They have a secret room in there castle that has the history of her journey coming to that point where she finally left the planet and never came back.. how many does she think she has to create to prove she can do anything and her cutie mark proves it?... who knows really.