[SeijiSensei]

Having never had a sibling, and having only one child myself, any opinions here are pure speculation. Chiyo-chan is obviously a good candidate, but I think I might prefer Haruka from Noein. She seems like such a good-natured, intelligent, and reliable girl.

Spoiler for Haruka:

Having the Dragon Torque as one's sister might also come in handy at times.

I just realized both these girls are only-children themselves.

Edit: You know, just about any anime girl or woman could be my "little" sister. Of these, I'd pick Nakajima Youko from Junni Kokki.

Spoiler for Yuuko:

I think being the brother of an Empress would have its advantages.