Possible MG Novel?

A general rule of thumb is to keep your characters just a little bit older than your audience. But then again, there are a few exceptions to that rule. 17 speaks YA to me, whereas 11 - 15 year olds generally show up in MG. But who knows, your agent or publisher might like it the way it is. (Hows that for being vague?)

To answer your first question: You can still have teenager behaviour and have an MC who is 12/13. Have you met kids that age recently? Man they are snarky! After having done readings for just those ages I have to say they already have the teen angst thing down. Also a lot of kids these days are really eager to grow up fast and want to behave more like rock stars than play make believe, so it can still be a shock for a kid that age to suddenly be thrown into the world of fairies. Remember, it all comes down to the character you have written, and not some general assumption of how all kids think. So you could have a very bitter 13 yearold boy who had to grow up fast for some reason let's say and so doesn't believe in magic etc, as opposed to a kid who just wants to escape to an imaginary world because his real world sucks so much. You are in control of what your character thinks and believes. I bet you could age the kid down to 13 and everything would be just fine.

Your second question. It's tricky. Of course there are no hard and fast rules and you can do whatever you want in the world of writing so long as you do it well. That said, I know as a reader I would be a bit pissed off if I had invested all this energy in getting to know a character only for the author to tell me, "And now you aren't going to be following her story anymore but this brand new character that you know nothing about!" Some authors solve this problem by switching back and forth between the POVs, meaning one chapter is one person, the next is another and back and forth, so then you care about both characters. If you still want to continue the way you started, maybe there could be a way of introducing your MC early on so we know you will be returning to him later.

Good luck with it all!

I'm with Toothpaste. It could work, if it's done well. But personally, I wouldn't. If you need to show what happens to the little sister, but the real hero is the brother, I think you should start the book with him. Switch to her POV if you must, but ground it in his character first. Or change the kidnapping so that somehow he is a witness to it, but powerless to stop it, so that we have the anguish from his POV but know what happened to her.