Family structure:
- Father: NAME_1, 43, balding, beer belly.
- Mother: NAME_2, 35, nice figure and rack.
- Son: NAME_3, 15, nerdy, dark greasy hair, unfit, shy, horny and creepy.
- Daughter: NAME_4, 13, pretty, blonde, slim, cute and girly.

Smutty and kinky story prompt, make it  open ended without a conclusion: One evening NAME_4 asks her mom about sex in general and masturbation in particular. She is curious and wants NAME_2 to explain how to do it and how it feels.
NAME_2 agrees to a training session of sorts in NAME_4's room. NAME_2 insists on letting NAME_4's brother be part of it so she doesn't have to explain it twice. She calls NAME_3 into NAME_4's room while NAME_4 and NAME_2 are getting undressed.
First NAME_2 shows on NAME_4's bed how she does it, showing and explaining everything and then NAME_4 joins in and applies what she learns.
Meanwhile NAME_3 is standing pantsless next to the bed and slowly masturbating, taking in the sight of mom and sis naked and spread eagle.
NAME_3 discreetly films the scene with his phone to enjoy later.