.... He claims that Kagayaki is his biological mother. While he might stretch the truth, he's never said anything that is outright wrong. Like the fact that no one has ever died in his training, because his save-and-load ability makes it so it never actually happened - it simply becomes subjective experience, which is apparently what counts for the most when it comes to leveling up your stats in this world. As he says in the very first chapter: "...the items and money you get after saving won't be taken back with you. Instead, you won't lose your memories and experience, so you'll become stronger after you die."
But I'm not sure if I can recall whether or not his wife was stated beforehand to be related to him. My impression was that she was part of the same group, just picked up and adopted by Kagayaki. But I could be wrong about that.
Either way, it wouldn't surprise me if Kagayaki was twisted that way - that she did set up those two to be together despite being siblings... And that it's one of the things Alec is miffed about.