Your sister is a girl or woman who has the same parents as you.

Sister is a title given to a woman who belongs to a religious community.

A sister is a senior female nurse who supervises part of a hospital.

You can describe a woman as your sister if you feel a connection with her, for example because she belongs to the same race, religion, country, or profession.

You can use sister to describe something that is of the same type or is connected in some way to another thing you have mentioned. For example, if a company has a sister company, they are connected.