Is Sandra's a common nouns

No, the Word Sandra is a proper noun, the name of a person. A proper noun is always capitalized.

The form Sandra's is the possessive form for the proper noun; a proper possessive noun. A possessive noun, indicated by the -'s, means that something in the sentence belongs to Sandra. Examples:

Sandra's mother invited us to dinner. (the mother of Sandra)

I borrowed Sandra's lawn mower. (the lawn mower of Sandra)

Your boots are just like Sandra's. (boots like Sandra has)