I see this error in people’s writing quite frequently, so I thought I’d do a post on this particular capitalization rule (which is easily found in the “Capitalization” section of this website).
Here’s how I teach the capitalize-family-titles grammar rule to my students:
Capitalize family titles (mom, dad, etc.), if you could replace it with a name and it works. Here’s what I mean: In a sentence like “We’re going to Grandma’s house,” you could have said “Ruth’s house,” and it still sounds fine, but if you had said, “We’re going to my grandma’s house,” it would sound silly to replace it with “my Ruth.”
I took Mom with me to the Justin Bieber concert. He kissed my mom on the lips, like she was Ariane Grande or something.
Why did I capitalize “Mom” in the first sentence but not in the second? Isn’t she the same mom?
Yes, she is. But try replacing it with a name in each sentence and you’ll see that the name only works in the first sentence. Let’s say my mom’s name is Cher for example:
I took Cher with me to the Justin Bieber concert. He kissed my Cher on the lips, like she was Ariane Grande or something.
(If you say it out loud, it sounds like you mean “my share”!) It’s clearly weird in the second sentence but okay in the first. Therefore, you’re only using “Mom” as a name the first time around.