Sans's brother is Papyrus who are skeletons
Sans has injected Perseverance in him which keeps him from dying/melting. This also gave him a 2x base Karma attack.
Toriel is the ex-queen of the Underground and ex-wife of King Asgore she had a son named Asriel and adoptive daughter named Chara until they were killed by humans
Gaster is Sans and Papyrus dad but was shattered across time and space by an experiment
Sans's left eye can glow blue/yellow