Question

Asking your sister for a cup of tea into formal or informal

Answer 1

Hyy dear...!!!
Asking your sister for a cup of tea is formal.
✌️✌️✌️☺️☺️♥️♥️♥️♥️☺️☺️✌️✌️✌️

Answer 2

INFORMAL

Anything (speech, dialogue, questions, chat) with the ones you know and are familar with (Family, Frends, Classmates and Jobmates) is Informal.

Since, your Sister is a part of your family, asking her for a cup of Tea is Informal.