You mean the theory who MC father can be Antonio's father and so Antinio 'll be MC's half brother ? (same hair color) but don't think is a right things.
Don't remember but what sais the old man in contract ? MC'father 've other childs and is in contact whith them... Don't think that old man speak about Antonio....
No, wait. You are mixing our theories. If I remember correctly
JJJ84 talked about a theory where Cordia was with MC father (being him her first love) and Antonio were born, and after some stuff she ended with Lucan. Others here said something similar (not with MC father) with Cordia being with another man before.
I said, regarding that old man (Steadfast) talking about MC father having other "childrens" (maybe not literally?) and being in contact with them) that maybe we has Mr Moustache (the man in the second room where Luna was while being kidnapped by M6) and maybe the other "childrens" were something like experiments, and that is what makes MC a target.
But, so far, no one talked about Antonio and MC being brothers (or half brothers) more than like an idea, but if you want to team up with JJJ84 with that then Ill probably need to make "teams" and we can start a gamble around the theories and the winner is our real DeLuca Serpent hahaha