Sans and Papyrus are brothers and reapers
Gaster is Sans and Papyrus dad but was shattered across time and space when he was in the void by chara, Chara went on a rampage killing some monsters by stealing what she thought Sans' true Scythe but stole a fake giving Sans the advantage to trap Chara, handing her to King Asgore to deal with
Toriel was the mother of Asriel and her adoptive daughter, chara.
Toriel would run away and hide behind a barrier she put up and controls/creates life