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.
"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!"