Sally-Anne from Melbourne, Australia, has a problem with family titles. She writes: I never know when to use capital letters for mum and dad when I’m writing dialogue (or maybe it should be Mum and Dad).
An easy way to remember when to use a capital letter is to determine whether or not the title is being used as a proper noun (as a name). If it is, then it needs to start with a capital letter.
So, if writing dialogue and addressing someone directly, it would be:
“How are you feeling, Mum?”
“Are you going fishing, Dad?”
"Put that bottle down, Aunt Sally. You drink too much."
However, if you’re writing about a family member and the title is modified by our/your/my/his/her/etc, then it doesn’t take a capital and would be:
“How is your mum feeling?”
"My mum is feeling down because of my aunt's drinking."
“Is his dad going fishing with your dad?”
"My dad is going, but his dad isn't."
The above rule applies in narrative, not just for dialogue:
My mum is fed up because my dad is always going fishing.