Capitalization of "sister" and "brother"

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.

Solution 1:

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".

Solution 2:

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".