I find Rainbow Dash and Scootaloo's relationship so interesting because Rainbow Dash is essentially fostering a younger version of herself. Their relationship to me reads as Scootaloo being RD's protege, similar to Celestia and Twilight. Scootaloo is RD's future, RD's legacy, and an extension of RD herself. RD and Scootaloo both share the fierce hunger to realize their dreams and accomplish what most ponies consider impossible. RD is what Scootaloo can be with the right amount of dedication, hard work, guidance, and inspiration. Scootaloo is the potential, RD is the dream. The sisterly aspect of their relationship is cute and heartwarming and it highlights the softer more caring side of RD that few ponies truly know. The fact that the paragon of the pegasi race, RD, is giving herself to an underdog filly like Scootaloo says a lot about both of them. And you know RD isn't doing it out of pity for Scootaloo. She does it because she sees herself within Scootaloo and perhaps something greater that even other ponies are incapable of seeing. Likewise, if you wanted to take a more cynical approach to their relationship you could say RD represents success where Scootaloo represents failure. Scootaloo being the representative of failure where one's dreams clash with their reality. This would hit RD especially hard because she sees herself in Scootaloo and in the same vein of RD's characteristic egotism, she would take Scootaloo's perceived failures as insults against RD herself. This perspective of their relationship makes Rainbow Factory's main conflict intriguing to me because it explores their relationship on a different level. It puts Scootaloo at the forefront of their relationship and it gives a more nuanced answer to "What if I don't measure up to the expectations she has for me?". The dichotomy of RD and Scootaloo and their relationship as a whole makes them a lot more interesting than the other sisters IMO. Its a shame they didn't get more episodes together.