If you choose to have a vision of the past when the mysterious lady offers it, it is revealed that Sybilla, Cassandra's mother, had actually killed Regis instead of the opposite which means that whoever married MC's mom and produced him and his sisters is not the original Regis. It can't be his brother Conrad either because he is still around.
So who is he? Well, Elin's portals provide us with a possible explanation. In every parallel world they visit, MC and his family existed in one way or another. I believe that the current Regis did in fact come from another world and replaced the deceased one. This also means that Cassandra is the only remaining heir of the original Regis.