Basically, the fandom started, then people got obsessed with Sans. Then people started shipping Sans with everyone. Alternate universes were made. People we're getting desperate because no one was good enough to ship with Sans... and then some on looked at an alternate universe and said, "Hey guys... what if we ship Sans... with himself..." And so Sancest began. Then people were like, "Hey guys... what if the Sans' had kids? And then we ship the kids with each other and THEY had ship kids???"
yeah you pretty much have the thing all figured out :0