When terms denoting family relationships are used as proper nouns (as names), they are capitalized. However, when the terms are used as common nouns (not as names), they’re not capitalized. Generally, there will be a possessive pronoun (my, her, his, our) or an article (the, a, an) in front of family titles used as common noun.
It’s easy to get confused about whether you should capitalize family names in your writing.