There's merit to this. Antonio's father shares the same "bad reputation" (based on the Capo's commentary) as the MC's dad. Ruthless ambition and a murderous temper are also not descriptive of Luna's dad
at all, so at minimum, Antonio's dad is certainly not the same as Luna/Grace's. Cordia's comment along the lines of "he looks so much like his dad" when kissing MC in the study also suggest that she and MC's dad were more than just former work friends.
This opens interesting possibilities - such as Antonio being the one behind the assassin attempts on the MC. A contract guy with mysterious value is a curiosity. A brother with very high potential is a rival and can drive one to do extreme things. The whole Steffran arc (4 whole main story events) always felt like it was too detailed for a minor side story to me. Perhaps the detail is
HopesGaming 's parable/foreshadowing of Antonio's end?
PS. Also conveniently clears the way for an MC/Isobel romance