Sister Japan has been seen twice with her brother (three times if you include the Know Your Tourist postcards) and this would imply that the two get along.
Sister Japan gets along easily with others due to her gentleness and good manners. She is friends with Sister America based on their shared love of male slash pairings and overall fascination with each other's culture. The former caused Denmark to take special notice of her, and to flirt with her by insinuating that he and Germany were an item. However, she responded by developing a soft spot for Germany instead, which he reciprocated. So it may be sour grapes that leads Denmark (with help from the Netherlands) to sabotage their budding relationship with constantly escalating "surprise yaoi" pranks. It is likely that she is friends with England as he invites her to his tea parties.
Sister Japan is depicted with the same caricatured Oriental features (slit eyes and a yellowish cast to the skin) as her brother. Her hair is black, very long and straight and evenly cut both back and front (bangs). She wears a long-sleeved, high-necked shirt with the design of the Hinomaru, a black skirt, white tights or knee socks, and Mary Janes.