you are Law's little sister.

When you came home, you saw some red-haired guy sitting on the couch watching TV and Law with him.

Law: "oh, so are you home already? This is my... well, let it be... err... rival, Eustass Kid. Eustass-ya, this is my little sis."

Eustass: "Dammit...Trafalgar! You could have at least tell me that you have a sister!!! I was going to kick your ass today!"

he said irritably

Law:"Eustass-ya, shut up, don't scare my sister, you're already scary."