a younger sibling
a cousin who is descended from an ancestor who is/was a younger sibling to oneself's or one's spouse's (such as a child of a younger sibling of one of one's parents or a grandchild of a younger sibling of one of one's grandparents)
a person younger than oneself but of the same generation
(formal) a child or a student

(familiar) pronoun used to refer to younger person of the same generation
pronoun used to refer to younger siblings or cousins descended from an ancestor who is/was a younger sibling to one's own or one's spouse's
(formal) pronoun used to refer to a child or a student
pronoun used to refer to the girl or woman in a romantic relationship

adj