My daughter's English text book has a note which reads if you are directly talking to a person about the only person in your house, it can be capitalized.
For example:
"Mommy, have you seen my tab?"
"What would I do with it? Ask your Sister. She's fond of playing on it"
I want to confirm the capitalization of 'sister' here. This is new to me, actually!
And yes, it has nothing to do with emphasizing 'sister' that many may think the reason of capitalizing. Say–
"Yes, last night I asked Mommy about it, but then she gave a reluctant smile!"
Best Answer
Your daughter's textbook is not quite accurate.
You capitalize such nouns when they act as names, as the ordinary form of address. "Mommy" is used that way in your second example.
But in your first example, sister is not a name, as its use with a determiner makes clear: it is a common noun denoting a relationship.
If the determiner is omitted, however, the noun is treated as a name, and it is proper to capitalize:
That sort of use of sister is rare, although not unknown; as Damien H says, Sister, capitalized, is usually the title (of reference and address) for a Catholic nun.