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I have seen the question on when to capitalize mom and dad, but I am confused why the rule would not apply equally to Brother vs. brother and Sister vs. sister.

I.e., if the rule is to use capitals when speaking of a specific mom, as in "my Mom", then should the rule compel me to refer to siblings as "my Sister" and "my Brother"?

It looks perfectly acceptable to capitalize parents, but capitalizing siblings looks extremely odd.

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In contexts like "You'll have to ask Mum" (as opposed to "your Mum") it seems quite natural to capitalise what is in effect a proper noun usage. The reason this would usually look weird with brother or sister is partly because often there are other siblings of the implied sex, so it can't really be an unambiguous proper noun. Also see Yes uncle,Yes Uncle –  FumbleFingers May 23 at 21:50
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@FumbleFingers - mother/mum, dad, sis, bro are capitalized if standing in for a proper noun, e.g. Sara, Sam, Sally, Steve. Not when used with my. However, these are only the grammar rules, and I don't object to how people want to do it personally. But as an EL&U forum, we should at least be aware of the convention. –  medica May 23 at 22:33
    
@medica: I thought that was effectively the point I was making. Not that I would ever expect to see “Wassup, bro?” capitalised, but there are certainly "exotic" contexts like Mother curtly asked Brother why the girl was in the hospital, but Brother wanted to hear her opinion about her before he'd answer where (the orthography, at least) is perfectly natural. –  FumbleFingers May 23 at 22:57

2 Answers 2

Mom and Dad are capitalized when they're used in place of names, e.g. Hi, Mom at the beginning of a letter or email.

Sister and brother are not usually used in this way. If you were to refer to a sibling using the relationship, it's more common to use diminutives Sis and Bro. If you write Hi, Sis, you should capitalize it for the same reason as Mom above.

You don't capitalize them when you're using them as ordinary nouns. So you would write "My mom was home and she made dinner for my sister".

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That's what I said My mom isn't using it as a proper noun, it's just an ordinary noun, that's why I didn't capitalize it. –  Barmar May 23 at 22:40

Sister and Brother are capitalized in the same way as Doctor -- when used as honorifics or titles. These particular titles, along with "Mother" and "Father", are commonly used by religious orders.

You wouldn't capitalize them (or "mom" or "dad") when used with an article or possessive pronoun. "my sister" and "my brother" are correct, as are "my mom" and "my dad".

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This is correct when they're being used as honorifics, as you say, but I think @Barmar's answer is closer to what the original question was trying to resolve. –  Joe McMahon May 23 at 22:22

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