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At that age, my little sister was really into Great Teacher Onizuka, Shonan Junai Gumi, Otoyomegatari, Gekkan Shoujo Nozaki-kun and Oresama Teacher. They're all really good, and I can vouch for them myself.
For the brother, something like Fullmetal Alchemist is also likely to be enjoyable for both, and would probably be very easy to find cheap or good-looking copies for. At that age I remember being quite into the Pokemon Adventure manga, which is good if he has any interest in that franchise.
