don't want to be rude but their not family in UnderTale sans explains there is more the just the universe that Frisk is in and what you thinks is Sans and Papyrous family is really versions of Sans from different universes.

The only family sans has is Papyrous and in a theory are related and cloned from W.C Gastor but they were cloned with "Determination"

that blue spark in sans eye is determination

auto correct I meant W.D Astor