Should you write Mom or mom? Is it more correct to write Brother or brother? While we all capitalize proper nouns, when do we capitalize those used to refer to them?
As a rule, you should only capitalize relationship names when they are used as a substitute for the person’s actual name, such as in “Don’t go see Dad today, he’s not in the mood.” Conversely, no capitalization is required when there is a modifier in front of the relationship name, such as in “My mom is still in Puerto Rico.”
These two examples should help make those rules even clearer:
“Where’s Grandpa? I want to see my grandpa!” (In this case, the Grandpa [in the first sentence] substitutes for the man’s name, while grandpa [in the second sentence] is simply referring to the man that I want to see).
“I never believed a single thing Uncle said. Anyone who believes my uncle’s stories is quite the gullible sucker.” I won’t explain this anymore – hopefully, you catch the drift.
A common mistake in capitalization are terms of endearment. Under no conditions should these vocative nouns ever be capitalized (e.g. “I love you, honey”). Everyday, I see plenty of writers guilty of these errors, so do watch out for them, especially since grammar software can only identify so many vocatives and will likely pass a few uncommon ones over.