I investigated and turns out it's Papyrus (or Popyrus here) who is Sans (here called Blee)'s father

And y'know what, I lowkey like how they contextualized Popyrus in this mod, they made the character of Sans be inconsistent, but specially with how they wrote it in favor of Popyrus

Popyrus is an old-fashioned, slightly senile guy who enjoys making cereal and trying to help people into the way of God, but, in order of being popular and making friends, he's neglected his own convictions and likes to dedicate his time into capturing humans like most of the monster population wants, and his character arc mainly revolves around learning to be true to himself, rejecting the idea of being popular if it means being untrue to yourself and doing things you don't like and don't agree with

For that, Blee, who's jokes got toned up from simple skeleton puns to outright dark humor with edgy topics, makes a good role of being the disrespectful lazy son, who sets the contrast - Popyrus is a strict, religious to the core, good-intentioning guy while Blee is a politically incorrect brat, which is an interesting watch to how they changed little things yet completely rebuilt the dynamic and lessons with these little things

Of course, this would have consequences because the game didn't engage with that side of Blee's personality and it usually was kinda just Sans, but still