@Fifths:
Because Scootaloo seems to be the sadder character, and there's a lot of differences between their role-models. We assume Scoots wants to fly because RD does, and talks about it all the time. We can't make the same assumption for Sweetie, as Rarity doesn't really talk about magic all the time. It'd be different, I think, if Twilight was Sweetie's role model, and not Rarity. It'd also translate to people possibly caring about Sweetie Belle more, because, on average, a lot more people like Twilight than Rarity. So, by extension of that, somebody closely related to Twilight (sibling), that's introduced earlier than the finale of the second season (don't interpret this as hating on Studio B for not bringing Shining up earlier, because I rarely ever mention my brother [to be honest, other people usually bring him up before me]) would be a lot more cared about than someone closely related to Rarity. Especially if they were lacking in something that defined Twilight.
I also think it's part of what everybody thinks their talent is. People think Sweetie is going to be a singer, which doesn't require magic (Sapphire Shores), while people think the Scoots is going to either be a racer or a stunt devil, something that benefits greatly from flight.
So, it's a bunch of things put together, along with Scoots getting (usually) the most love of the CMC, since she seems to be a 'sadder' character.
As for the Angel thing:
First of all, Rule 1 of the Pony Thread: Don't talk about Derpy. It's like Rule 2 (DON'T talk about Derpy).
Second of all, he acts more like a toddler than an abusive spouse. When his 'mom' is in distress, he acts kind and loving, but he gets upset when she gives attention to somebody else. He also acts spoiled and demands what he wants, because he can. Fluttershy is a character that fits as a more lenient mother for Angel because she's timid, which is an open door for a toddler. However, I think it's hinted that she kept the toughness Iron Will gave her, so Angel isn't as demanding, which is somewhat proven by his appearances in Hurricane Fluttershy. Not just when he was comforting her, but also when he was cheering her on (after getting tossed away by a tornado) before she started flying.