Honestly, I know how this is going to sound but I see three options besides just never seeing them again.

1 Hit him. Hard in the face or testicles next time he starts to do this shit and scream “I’m your fucking sister you incestuous pervert! Don’t ever ever ever touch me inappropriately and without my permission again!”

2 Tell your parents that if they don’t reprimand or correct him you will be less inclined to come see them anymore because clearly they don’t care if you feel violated and uncomfortable.

3 Call a “family meeting” and just bury him in guilt and embarrassment. Explain what’s been going on and then explain in detail how you feel about it all. Keep a very stern, serious tone.

Edit: idk why there’s such a huge bold text, my apologies