pediatrics, either as a course of study or a department of a hospital

have a disease; disease / being stiff and not able to move / difficult / suffer / make one suffer

disease, illness / a bad habit

disease, illness

someone else's younger sister

sworn younger sister

younger paternal half sister

a wife's sister

(archaic, men's speech) an intimate form of address for a female by a male: "my sister"

(formal) we, us