Here's Rosetta and her bestest friend in the whole wide world, her cousin Sweet Sediment! Rosie Pie & Sweetie Pie!
Sweet Sediment (her name is a play on words for 'Sweet Sentiment' and also rocks) is the eldest daughter of Fancy Pants, Maud Pie and Fleur De Lis (biologically, she's Maud and Fancy's daughter). She is quiet and sweet, very sophisticated and just an overall well-bred young lady. She doesn't bend to the trends of her hometown rather she incorporates what she wants and does her own thing the rest of the time. She's a darling in Canterlot due to Fancy Pants and Fleur De Lis being the highest of the social elite and Maud being well known/beloved in academic and political circles. She doesn't particularly care for most of the important circles in the city but she knows how to play the game as well as anypony.
She's very close with all of her parents as well as her Aunt Rarity and Auntie Pinkie Pie — as Maud and Pinkie are sisters and Fancy, Fleur and Rarity are very close friends outside of their wives, the two families often spend time together. They're a tight-knit family.
Her talent is her ability to make beauty shine outward (as represented by her geode Cutie Mark) and she does that as a photographer — she's also an actress on the side and does a lot of lowkey, heavily dramatic theater. She's not famous per se but she has established herself in that particular scene as a very talented and passionate performer and she has a decent following — she's considering branching out in the future to film projects.
She has a tendency to, like her parents, shut others out — whether to protect her reputation and keep herself controlled or just simply because she has her limits on what she chooses to express. She can also come across as icy and cold like Maud can and there are those who see her as stuck-up due to Fleur and Fancy being as wealthy and socially powerful as they are. She's actually quite pragmatic and down-to-earth. She enjoys a high-class lifestyle and doesn't want for anything but she has a level head and doesn't look down upon others as long as they're good.
She also has a well-hidden mischievous side, a love for the shadowy and macabre (which she'll show a glimpse of from time-to-time, if she wants to freak somepony out) and could bench press a truck. Her voice actress is Liz Gillies.
She and Rosetta are only a few months apart in age and as such are the closest of all the cousins in the Pie family. They make sure to see each other a lot and they always love spending time together. Only child Rosie considers Sweetie to be her sister while Sweetie actually has siblings and is like… 'no… I actually like to be around you for more than a few minutes…'