A sister is a woman or a girl who shares parents or a parent with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to refer to non-familial relationships. A full sister is a first-degree relative.

A sibling is a relative that shares at least one parent with the other person. A male sibling is a brother, and a female sibling is a sister. Somebody with no siblings is an only child.