Due to this being my first critique I will be following a cookie-cutter template to state my opinion.
Vision: I personally find this art reflects a resonating theme of kindling a burning ember into a fire of emotion that helps illuminate the future. Or, simply, learning to care and love. It's a theme that can pluck the heartstrings of many if presented right, and you, sir (or mam), have done so in a beautiful way. You have wowed your audience with the theme and have successfully delivered it.
Originality: Your style is quite interesting and refreshing compared to other drawings for MLP. The style of the ears are something that I see 'once in a blue moon', and even the manga style eyes are somewhat new. However, I also need to add that the subject of Scootaloo's and Rainbow Dash's relationship has already been done multiple times and is becoming a little stale. Whether it's drawings of Rainbow Factory or harmless ones like this, there are hundreds of them flooding through many websites.
Technique: As mentioned earlier, I enjoy your style and admire your sweeping brushstrokes; however, there are a few things I want to nit-pick on. First, there is a sliver of silver in Dash's tail that is uncolored and the outline colors do not match those in the show. Second, Scootaloo's wings are entirely disproportionate to the rest of her body, try and enlarge them some. Third, I personally don't like the bit of fur that connects the tail to the body. There are also some things that you did get right (i.e. Dash's CM is in the right spot/has right colors, overall mane and tail styles/colors, and giving Dash a believable snout).
Impact: What can't you say about this that has already been said in the comments? This absolutely melts our hearts by seeing the love and joy shared between the two. In short, I can't handle all the feels. I almost felt tears in my eyes from the absolute soul wrenching connection that is conveyed by this piece. Great job, A+, gold star, or whatever kind of medal you want for making half of the comments section either squeal in delight or tear up from the beauty of it all.
What sells this pic for me, besides the fact that you did a pretty good job of capturing the art style of the show? The concept of this picture. While we know that Scootaloo DOES have a home (as seen in "Flight to the Finish"), we still haven't seen her parents, or know what kind of relationship she has with them. Thusly, the idea of being officially adopted as a little sister by her idol might just make the little filly's day, and then some.
Seriously. The look of tearful joy on Scootaloo's face just gives me all sorts of feels, along with a melting heart.
RD: "You know how we've always called each other sisters?" SC: "Yeah?" RD: "I... I couldn't stand saying that, and knowing it's not true. So..." SC: (Scared) "So what?!" RD: (This scene) "So I went ahead and-" (She's cut off as Scootaloo embraces her, tears streaming around her enormous smile as she half-cries and half-laughs) RD: (Smiling) "Come on, Squirt. Let's go home." SC: (Stares up at her) RD: "Our home!"