Option 1: My mother had an affair with someone and hid her pregnant belly to my father for 9 month then proceeded to give birth in total secret and give him to orphanage (which surely did not happen cause my mom is the kind of person you can trust with your life and i believe she woulda told me abt a secret brother)
Option 2: my dad had sex with a prostitute and she couldnt keep the child
(hypotetically this could have happened but i doubt it)