a sister is a female sibling, meaning a female person who shares the same parents with another. In terms of grammar, it is a noun that can function as a subject or an object in a sentence. It should always be preceded by a possessive adjective (her, my, your, etc.) when used as an object.
when used as a subject, sister usually has no pronoun preceding it. When it’s used as an object, however, the possessive adjective should be included to state ownership or relationship. In proper English, sister should never be preceded by an article (e.g. a or the).
1. My sister always laughs at my jokes.
2. Your sister is so beautiful.
3. We visited our sister in the city.
when referring to a person’s sister, never use the article 'a' (e.g. a sister). Additionally, it’s important to note that sister is only used for biological siblings. If one is referring to a close friend, the terms 'friend' or 'pal' should be used instead.