In the other room you hear your mom and sister talking:
Ew, Mom... He did it again! He keeps getting all this STUFF all over my panties. I...I don't know what it is. What is that? EW! Mom, make him stop or something.
Your mom and sister sit in front of you, cum-covered panties in hand, and tell you that you have to stop. Your sister is disgusted beyond belief, and she's well-aware that you have no intention to stop. But Mom has an idea. You're going to jerk off in front of them.
Your sister smiles, "Oh...WITHOUT looking at our panties." They put their wet panties behind their backs, and tell you to take your cock out. You're hesitant, and completely embarrassed, but they're persistent. Once you muster up enough courage to finally pull it out, you're rock hard. Just the thought of cumming all over your sister and mom's panties is enough to get you throbbing.
"See? Thoughts. Now you gotta do it without seeing them," Mom says. They encourage you to wrap your hand around it, but still shy, you take your time. "Well, don't just hold it," your mom and sister laugh. You're reluctant, but their soothing voices coax you into stroking.
With their encouragement, you find yourself oozing precum. They giggle, and notice your hard cock throbbing and your veins bulging. You get even more turned on when you realize how intrigued they are. They want you to feel good. Even your sister wants to see your cock.
Once they see you pumping harder and faster, and ready to erupt at any moment, they start a countdown. "In your hand, no panties," Mom reminds you. They playfully count down from 10, and once they hit 1, you explode. "Perfect. Now that is the way you will do it from now on, son" Mom says. "Yeah, I better not find any more of my panties with your cum on them," your sister snaps, and then smiles at you