Your sister is going off on how your Mother has so little faith in the both of you. Mostly your sister, who she thinks is a big slut requiring birth control. However, your sister is a virgin, like you. The both of you haven't had that experience, and it's kind of funny that your sister was made to take birth control.
She finds it a little less than funny though. Her tits hurt, she feels fat, and her pussy doesn't feel the same. She tells you that it is kind of funny though, that your Mother put her on the pill so she wouldn't have sex, but all she wants to do now BECAUSE of the pill is have sex. And what better way to get her back then to have YOU fill her pussy with your cum? Mom would flip!