Harry introduced Kara as "best friend" in Diagon Alley, and Hermione calms Daphne by referring to the best friend relationship (but why would that calm Daphne, don't best friends turn into lovers at times?). But whereas we saw Harry thought of Kara as a sister, there doesn't seem to be any reason for Hermione and Daphne to ("They had a three year age cap but saw none of the usual fights older and younger siblings had.").
Why would a 16-year-old girl (she was 15 when Harry was 10 in chap. 1 - a 5 year not "a three year age gap") steal an 11-year-old boy's shirts to wear them? She wouldn't fit them, would she? Harry must be very, very big for his age (or else Kara is extremely small, but then Hermione wouldn't surmise an age of 14 or 15).