兄弟姊妹，也作兄弟姐妹，又称手足，指有相同父亲和母亲的人。较自己年长的男性为兄，女性为姊；比自己小的男性为弟，女性为妹。最年长的称大哥或大姊，余下按排行次序称呼。两人或全部皆为男性为兄弟，两人或全部皆为女性为姊妹或姐妹。年长男性与年幼女性为兄妹，年长女性与年幼男性为姊弟或姐弟。

同父异母或同母异父的兄弟姊妹于英语称为半兄弟姊妹（Half sibling），中国古代把同父异母的兄弟姊妹称为异母兄弟姊妹，把同母异父的称为外兄弟姊妹。父或母再婚，继父或继母之前与他人所生的子女称为继兄弟姊妹。

A sibling is one of two or more individuals having one or both parents in common. A male sibling is a brother, and a female sibling is a sister. In most societies throughout the world, siblings often grow up together, thereby facilitating the development of strong emotional bonds. The emotional bond between siblings is often complicated and is influenced by factors such as parental treatment, birth order, personality, and personal experiences outside the family. However, there are cases where siblings grow up in separate homes, in different environments. It is known that both nature and nurture figure in development; researchers are attempting to ascertain just which one plays the larger role.