You should capitalize these when referring to your own relatives: Hello, Mother. A good rule to follow is to capitalize them if they are used as proper nouns. If used as common nouns, don’t capitalize as in: We honor all mothers in May. In other words, capitalize words such as “Mother,” “Father,” “Grandmother,” “Grandfather,” “Son,” “Daughter,” and “Sis” when they are used in place of the person’s name. Do not capitalize them when they follow possessive pronouns such as her, his, my, our, your.