Alright so first definitely tell your sister. But if you’re too afraid I have a different option. LEARN BOXING. Or any form of martial art to defend yourself and your sister. This Kyle kid is a bitch and probably wouldn’t do this to anyone his age bc he’s afraid. If you learn how to fight or defend yourself this kid will run away like hell. Ok it’s hard to find classes due to COVID but hell YouTube taught me a lot about fighting when I was learning. Ik this is such a generic response but I was in a situation like yours (little different but along the lines of someone bigger than me picking on me). I’m a decent sized dude so not a lot of people can intimidate me with their size but this dude was straight jacked. He beat me up once and after that I was just like fuck it and learned kick boxing. After that I kicked his ass for calling me a racial slur. Fighting should always be last resort but he already physically assaulted you, he won’t be afraid to do it again. Please learn to defend yourself and tell your sister the situation at hand.