What if your mommy, your big sister, your little sister, or all of them together got to decide how big your cock would ultimately become? Would they tease you and threaten to make you small? Would they make you really big, for their own selfish reasons? Would they ask their friends for advice? Would they bargain with you? What would you do or give to have a big cock?
I guess there’s no reason this wouldn’t work with your daughter(s).