Sans and Papyrus are brothers
Sans loves puns and is technically the prince?
Papyrus and Eras hates puns unlike Sans, Toriel and Berlin
Papyrus is the king of monsters which technically makes Eras princess and Red the queen but Eras really doesn't mind or care too much
Asgore and Toriel are siblings
Their dad (Asriel accidentally fell into his creation and was shattered from time and space and is the mysterious man that Berlin sees)
Red still loves her swords and to spar and sometimes likes puns