by Jill Williamson
If your character is addressing her mother, father, sister, brother, aunt, uncle, cousin, or grandparent and first uses a pronoun (my, her, his, their, our), the family title must be lower case.
Because my mother said so.
I love our dad, don't you?
That car belongs to his grandpa Bill.
Greg, this is my aunt Shellie.
If your character is addressing her mother, father, sister, brother, aunt, uncle, cousin, or grandparent, and does not use a pronoun, the family title must be capitalized.
Because Mother said so.
I love Dad, don't you?
That car belongs to Grandpa Bill.
Greg, this is Aunt Shellie.
Terms of endearment, enmity, or indifference
The same rules do not apply for terms of endearment (honey, baby, sweetie, dear, darling), enmity (jerk, punk, idiot, stupid), or indifference (dude, man, buddy). All such terms should be lowercase.
Terms of Respect
Titles of honor are capitalized (His/Her/Your Majesty/Honor/Highness/Excellency).
All other general terms of respect are lowercase (sir, ma'am, miss, madam, my lord, my lady).