Sans has a brother named Papyrus but Papyrus was Erased along all of the Monsters except Sans and Frisk and unfortunately combining Sans and Gaster to G!Sans (Just Sans) When experimenting on DT... This event is Known as SHIFT
Gasters Wife Arial was killed during a human
Gaster is the dad of sans and papyrus
Sans idea was to bring Frisk to the core to give her Determination to bring back everyone back and succeeded but left sans still fused with Gaster and Papyrus a bit corrupted