When would "mom/dad" require capital letter in writing? [duplicate]

When it is their name, Mom and Dad would be capitalised:

Dear Mom and Dad,
I am just writing to let you know, that although everyone has a mom and a >dad, you are my special mom and dad. So, Mom, I just want to say ‘brava!’; and Dad, ‘bravo!’. I’ve just been telling Sis, that Mom’s new coat is so cool. XX

Here's the commentary from an exercise from BBC English on Line

...to have a roast chicken lunch with my mother. I can't wait! - mother does not need a capital 'M' as it is not being used to replace her name. If I said, "I am going to lunch with Mum", it would need a capital letter, but "I am going to lunch with my mum" does not.

We should capitalize these words if they are being used as the name of the person. You can capitalize these when referring to your own relatives: Hello, Mother.

• When you use mom/dad in general meaning father/mother, it's a common noun. So do not capitalize them when they follow possessive pronouns such as her, his, my, our, your. (my mother ~ my mom )
• A good rule to follow is to capitalize them if they are used as proper nouns. (dictionary.com)