Is sister's a common noun

Answer 1

Yes, the word sister's is a common, possessive noun.

The noun sister is a common noun, a word for any sister of anyone, anywhere.

By adding the apostrophe s ('s) to the end of the noun, it becomes a possessive noun, a word that shows something in the sentence belongs to that noun. Examples:

My sister is a lawyer.

My sister's office is next to the courthouse.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• Sisters of St Francis, Rosebud, MT
• Sister Bay, WI (pop. 886)
• Middle Sister Wines, Napa, CA
• "Sister Act", 1992 movie with Whoopi Goldberg

Answer 2

Yes, "sister's" is a possessive noun indicating ownership by a sister. It is formed by adding an apostrophe and an s ('s) to the noun "sister."