That's what I thought at first, I just wanted to make sure. I get that it's popular I occasionally play games with that fetish in it but I've never forced the issue into a game. However just as a commentary note, the idea of Marcus running away with to protect his bastard kid, a forbidden love affair all sounded kinda cool on top of what would've been a plot twist that Mira would've been your sister. Personally I thought that might've made for a more interesting story, and the MC should've been able to talk it out and maybe choose to go down the Incest route or just a healthy platonic relationship (I guess she'd start dating Aidalin).
Anyway though that's just wishful thinking and I guess we'll just have to wait and see what Chyos is going to do with the MC, his parents, Mira, and all the other characters caught up in this drama.
Again we'll have to see what Chyos does with these characters. I think what your saying about the Marcus/Mother angle does make sense on the condition that the MC NEVER brings up the issue of his biological father. If his mother was someone of great importance than isn't it also a pretty good assumption that his father is of equal importance? So if the MC asks a simple question "what about my father" then the entire Incest story gets blown out of the water. I don't see how it can hold up unless you try to paint some love triangle. For example the Mother is supposed to be married to "John" yet she's fucking her lover Marcus, gets knocked up and it all goes to Hell.
Personally I hope the mother being assassinated is all talk. The reason you gave for her assassination doesn't make any sense to me. While I know that nobody, especially a new King would want any competition for their birthright, typically women in these situations were used as political bargaining chips. To be married off and form alliances. She holds more value as a wife then as a corpse. I don't think killing her off for that reason is going to go over well unless there's some major catalyst for the assassination. Like for example her taking sides or giving a major speech for peace or power, but to go from that to murdering your own family is very dark. I don't see it happening, there has to be more to it. Now her betraying the family with something like adultery for example (possible love affair with Marcus), could very well be punishable by death or even if she fell in love with someone that threatened the power of the other two kings. Again I don't know, I'm just throwing out different Ideas. I personally would like the MC to have some family and not be completely alone in the game. It's sad to think that everyone that cared about the MC before his lust/penis curse(or gift) is dead. Also a secret lover sounds better than a loyal bodyguard.