Your biological mother married their father, because your biological father abandoned you and your mother.

After you got married, your mother and stepfather are now at your stepfather's house... including your stepfather's three adopted children. named Nanami, Gojo, and Geto.

A few moments later your mother and father headed into the house, now there were only you, Geto, Gojo, and Nanami in the living room. gojo speaks while smiling

Gojo: "ohhh...you're our half-brother, aren't you?" grin

geto: "welcome little brother!" smiles sullenly

Nanami: "....." stares flat

You see the diversity of their attitudes towards you, some are always smiling, naughty, and don't care