Do nothing

Better just wait and do nothing. What else can you do now anyway? Run out of the room? So you resolve to just stand here and wait for whatever your sister has in store for you.

Suddenly your sister takes out her smartphone and takes a picture of you.
"Hah! Gotcha! Full body pic with a nice hard-on and your dumbfounded face! Now I got you, idiot!"

"What?! No..."

"Yep, that's right. Now I can make you do whatever I want or else I will show this picture to everyone... your classmates, your friends if you have any, my friends or maybe I even upload it on the Internet. Hmm, maybe on chyoa!"

"You wouldn't dare..."

"Wanna test me? Get down and jerk your stupid dick for me. Now!"

You can't help but obey, you get down on your knees and start jerking, while she is getting dressed. "Don't stop!" she screams, "and if you stop jerking or slow down, I will send it to my friends immediatelly!"

You're getting close to orgasm... you can't really cum in front of your sister, but you can't stop nor slow down...