There were once three brothers called Hades,Posiden,and Zeus
They each drew sticks to decide their domains.Zues got the sky,Posiden got the seas,and finally Hades got the underworld.Everybody liked what they choose except Hades who felt cheated
So Hades cursed his brothers children so that they could never come into his domain.Posideon beleving Zues had done this cursed Zues children so they could never come into the sea.So of course Zuez did the same thing to Posideons children,he cursed them so they could never come into the sky
So when Hades killed his bothers children just for being born both bothers (Zues and Posideon) cursed their bothers children.
So in the end neither bothers children were able to go into their uncles domain.