Is sister possessive noun or a singular possessive noun?

Answer 1

The noun sister is not a possessive noun. The noun sister is a singular, common noun, a word for a person.

A possessive noun is a word that indicates that something in the sentence belongs to that noun. Possession is shown by adding an apostrophe s ('s) to the end of the noun or just an apostrophe to the end of nouns already ending with s (s'). Example:

Singular possessive: This is my sister's room.

Plural possessive: Both of my sisters' husbandswork at the university.

Answer 2

"Sister" is a singular possessive noun when it shows ownership or relationship to another noun. For example, in the phrase "my sister's car," "sister's" is the singular possessive form indicating that the car belongs to one sister.