Is twin a proper noun

No, the Word 'twin' is a common noun, a Word for any twin of anything.

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

• The Olsen Twins, Ashley and Mary Kate, American actors and fashion entrepreneurs
• Minneapolis-St. Paul (Minnesota), The Twin Cities
• Twin Falls, Idaho
• Twin Lakes Lodge, Hurley, NY
• "Twin Dragons", 1992 Jackie Chan movie