In My Little Pony: Friendship is Magic, the Cutie Mark Crusaders (i.e. Scootaloo, Sweetie Belle, and Applebloom) apparently live with their big sisters (i.e. Rainbow Dash, Rarity, and Applejack). Applejack seems to live with extended family on the farm. However, the rest of the ponies strangely don't live with their parents. We've only seen Rarity & Sweetie Belle's parents of these three, which is kind of odd. Is there some explanation for why they live with their big sisters?
I think the premise of this question is incorrect.
- Scootaloo does not seem to be the sibling of any pony who has been named so far. In Sleepless in Ponyville she explicitly hopes that Rainbow Dash will "become like my big sister", which makes it clear that she and RD are not sisters, and also strongly suggests that she doesn't have any real (but unseen) big sister she could be living with. Flight to the Finish briefly shows Scootaloo's home, but doesn't reveal who else lives there.
- Sweetie Belle definitely lives with her parents most of the time; it was only during Sisterhooves Social that she lived with her sister Rarity, and from the way they clashed it's reasonable to imagine that they do not normally spend a lot of time together. Ponyville Confidential and Princess Twilight Sparkle, part I appear to suggest that Sweetie Belle is now hanging out at Rarity's house more often, but One Bad Apple has a scene in Sweetie Belle's bedroom after an establishing shot that definitely doesn't show Rarity's home.
- Apple Bloom, as you pointed out, lives on a farm with her extended family, as does her sister Applejack. It's more that they both live with Granny Smith, really :)
The only things I can point out (and one's not lock-on-target on subject) are: When they were in the talent show together, the three following others were there to congragulate them (Rainbow Dash, Applejack, and Rarity).
The other, more off-topic, point is that they probably knew each other before, because in Episode 1 they were crouched up and wimpering beside each other.
Applebloom, Sweetie Belle and Scootaloo should have met (in theory), long before Call of the Cutie. Say that Sweetie Belle and Scootaloo had been friends before they met Applebloom. Applebloom and Sweetie Belle are the younger sisters of Applejack and Rarity respectively. In Celestial Advice (Season 7), it shows that Rarity and Applejack had met before they met Twilight. Presuming they were friends as it seems like they were. You would think, the family would have come up at least once. It is something that they have in common, a younger sister. The older sisters are friends, you think at least Applebloom and Sweetie Belle would have met at least once. Therefore Applebloom and Scootaloo would have met each other through Sweetie Belle, once Sweetie Belle and Applebloom were introduced by the two older sisters (AJ and Rarity). Therefore the Cutie Mark Crusaders would have met before the episode Call of the Cutie, when they were actually introduced.