"Baby Bones" An Undertale Fanfic
Sans has a good life. A caring father, an adorable baby brother. Things couldn't have looked better. But soon after the two started school in snowdin with the other monsters instead of being home schooled, disaster strikes. Will Sans be able to get through this newfound relization unscathed?
This story goes by the theory that gaster is the father of papyrus and sans. Though the theory is incomplete since no one knows who the mother is. For all we know sans and papyrus could have been made in Gaster ' s lab since they just "appeared one day"
No one really knows, but it's fun to debate about.