Step-brother's taboo bargain

My sexy step-sister has been driving me insane. She is always teasing me by walking around in cute skirts and she never seems to wear a bra around the house. Every night I have to listen to her be drilled by her boyfriend. She's such a sexy little slut. She promised her boyfriend that she would buy him tickets to a music festival for his birthday. She always procrastinates and I know she is going to wait until the last minute to buy the tickets. Naturally, I want to take advantage of the situation. I'll buy the tickets ahead of time, and then cut the internet at the last minute when she is trying to buy them. She will have no choice but to negotiate with me for the festival tickets. She will be so desperate that I will be able to demand whatever I want from her tight body.