If your older sister tells you not to tell mom and dad about the party while they're out of town you keep quiet. Then when your sister is partying you get a camera and the record the footage. Then when your parents come home tell them that your older sister/brother threw a part if they don't believe you show them the video. If they believe you wait till they bust her. Then if she denies it show them the footage. You might get beat up and get bloody but who cares you just trolled a older sibling!;)
Grab the phone, run to your room, and call 911 and tell them to call the hospital to tell THEM to tell your parent's your sister's having a party with harassment.