The word ‘sister’ is not typically considered a proper noun. Proper nouns are specific names that identify unique entities, such as ‘Jessica’ or ‘Mount Everest.’
‘Sister,’ on the other hand, is a common noun that refers to a female sibling in general. It does not specify a unique individual but rather a general category. However, if you use ‘Sister’ as a title or in a specific context (like addressing a nun), it can be capitalized and treated similarly to a proper noun.