Oh my god. You haven't had ice cream! *drags both fell brothers to the ice cream palor* Under fell sans: Wat. Under fell papyrus: Wat. Ice cream is needed!<<< *drops tubes of ice cream in their lap* EAT AND BE AMAZED!
An interesting idea. This would explain Sans feeling of responsibility to the underground to stop Frisk and be a good older brother. Sans would feel like he needed to be strong like his father and stand up to the bullies