Sibling as Parent
The parents are either deceased or otherwise unable to take care of the kids, so they are being raised by an older sibling.
I'm going to use a couple of movies / TV shows as the examples, because try as I might, I can't come up with good book examples. (I know they're out there, though! If you can think of any of them, please mention in the comments, and I'll get some book examples up here.)
So in Lilo, her parents have died, and her older sister is taking care of her. In Carly, her mom is deceased (I think... I'm not sure I've seen an episode where it has said.) and her dad is in the military and always gone. So she lives with her older brother.
- Let's be honest: siblings generally don't keep as good of an eye on a kid as a parent does. They just don't. And voila! This makes for kids who have much more freedom to go off and be heroes than they would otherwise.
- It adds an element of sibling dynamic that you don't usually get to explore. The sibling has to not only fill the role of the sibling, but also fill the role of parent, which gets things interesting.
- Sibling parents tend to be over/underprotective on random and unpredictable things, making them flexible in both good and bad ways.
- It's not overdone. (Ha! See above inability to list books... ;))
- Coming up with a way to make the circumstances in which the kids would be being raised by a sibling believable can take a lot of work.