A question about mental handicaps

So, I'm after thoughts here. I'm writing about a hero who's a kind of battlefield commander in a fairly silly but violent fantasy war. Because he's from a fairly brutal country and has no other family he keeps his sister close to him at all times, even though he's aware that he's putting her life in danger. He gets off with it, because the army he's in is very poorly run.

Im of two minds whether she should be younger than him, or older than him and suffering from some kind of mental handicap. The first would be simple but cliched, and the second would be awkward and complicated, but isn't that how life kinda works? It would also allow me a lot of room to explore his character more.

Thanks in advance for any thoughts from you guys.

Okay, two questions. Firstly, what kind of warfare are we talking about? Swords and shields? And secondly, what kind of mental disorder?

Well no, the tech level is roughly cold-war era with a bit of a crazy science angle, I guess. And I was thinking of making her sort of like a child in an older body, if that's what I end up going for.

I guess it depends on how much of a role you want her to have. If you want her to be a huge part of his life, then the older handicapped sister might be an interesting angle to take; it would definitely be an exceptional situation for him to be in, looking at it both as a piece of literature and relative to the other characters around him. But if you aren't planning on touching on their relationship much; leave her younger. Of the two options, that would probably be a lot more manageable.