Taboo Family Blackmail

Sister fucks dad twice while you (Brother) spies through open door. Sister notices and closes door. You come into sisters room to confront her about her fucking dad. She says it was to save the family, you don't care and blackmail sister. You say if she doesn't fuck and suck you, you'll tell mom she's been fucking dad. She reluctantly does what you say, enjoying it so much she calls herself your slut!