With Pinkie, her and her sisters tend to just be a bit extreme in their personalities (exhuberent for Pinkie, stoic for Maud, aggressive for Limestone and shy for Marble.)



Though there are some implications that Pinkie's personality came from her grandmothers (Grandma Pie, likely her paternal grandmother and thus married to Feldspar Pie and Nana Pinkie, Cloud Quartz's mother by default and probably where her name sake came from.)



As for Rarity, I am thinking she cultivated her artistic personality and such to stand out mroe from her parents, especially if she wanted to get into high society and whatnot.