She gets a lot bigger

Sarah reaches under her skirt and pulls down her panties, kicking them off into Doug's face as she straightens back up. He stares as his little sister's skirt starts to tent up as she becomes hard. Her pole quickly outgrows the rest of the tent, putting her expanding member in full view. Her cock and balls have the same proportions as before, but their relationship to her body has greatly changed. She blows past a foot and a half long, the growth finally slowing down as she approaches two feet. When it stops one of the girls takes out a yardstick and measures.

"Two foot, four inches!"

All of the girls are speechless until Jessica recovers enough from the shock to speak, "Wow, I had to do seven or eight rounds of milking my dad before I got that big."

Sarah just eyes Doug's ass. "I think I need to take this thing for a test drive." She stumbles a bit as she approaches his backside, her new equipment throwing off her stride. She pulls out the buttplug with a wet *splorch* and before any of the cum still inside can escape, quickly replaces it with her massive cock.

"Oh wow, this is amazing! Every inch is even more sensitive than before, and there's so much more of it now!"

One of the other girls removes the gag and proceeded to face-fuck Doug Meanwhile, Sarah, still adjusting to the extra stimulation, is already cumming. Even though Doug had already felt near bursting, he manages to take Sarah's increased load, and as the afternoon goes on, the loads of all the other girls.

The bell rings for the end of the school day, and Sarah decides it's too risky to keep Doug at school, now that she and her friends have seen what his boycum can do.

Sarah rolls Doug out to the curb, where she gets jealous stares from all of the girls and mothers, and he gets horrified looks from all of the boys and fathers. After what seems like an eternity, their mother rolls up with the family minivan.

She takes a shocked look at Doug, "Sarah! Is this what you've been doing with poor Dougie?" She quickly changes her tone as she reminds herself that he is Sarah's property now and she can do what she wants with him. "Well, roll him around back, I'll fold down the rear seats."

After slamming the tailgate shut she finally notices in shock the improved equipment now dangling out of her youngest daughter's skirt, even larger flaccid than her own gets when hard.