Eien no Nohara

Kazuhime is a novelist, and her younger brother, Nitaro, is a high school sophomore who runs the household. The Furuya family has been living without a pet since the death of their old dog Kaki, but now a puppy named Mikan has joined the family! However, Mikan is distrustful of humans, and Nitaro will have his hands full dealing with the new dog!?

(Source: Shueisha, translated)

Note: Chapter count includes 9 extras. Winner of the 15th Kodansha Manga Award in the Shoujo category in 1991.