Oh, this is really cute, and the background is very realistic, and just to lovely for words. I like the subtle sunset/twilight details, and the stars coming out in the sky. The clouds look great, and realistic to. Scoots & R.D. look great to, and I like how their hair/manes are blwoing in the wind.
I assume you're refering to "Sleepless in Ponyville" episode, since RD and Scootaloo ain't really sisters... then again, in MLP I overall don't seem much family resemblance in siblings (Applejack, Applebloom and Big mac look nothing alike), the only exception being Rarity and Sweetie Belle... I do like the art, background especially. Good work.