One of the surviving elf wants to bring back hia sister to life to start the race anew... That is incest and I did not explore that line.
Err... Not necessarily incest, as Kasimir isn’t the only surviving elf. It’s entirely possible that his intent was “just” to volunteer her for a breeding program, but not to participate in it himself. That would
still be deeply misogynistic, and I suppose there would need to be incest by the second generation, but I didn’t get the impression that he wanted to sleep with her.
On the subject of incest though, it’s also worth considering Izek and his incestuous feelings towards Ireena. Personally, I excise this from my version of CoS. I actually soften Izek quite a lot in general. I have him be one of the PCs’ long-lost brother rather than Ireena’s - preferably a Tiefling PC if anyone is playing one, though that’s not entirely necessary. If it works with any of the players’ backstories, I have their family be tied to Barovia (specifically, I tie them to the Durst family so that they have a claim to the Old Bonegrinder), but fled with a group of Vistani when Izek and the PC were babies. If it doesn’t work, I just have it be that Izek got lost at some point and taken to Barovia by the mists. Either way, he got taken in by the Burgomaster as per the adventure as-written.
I keep the part where he dreamt about his sibling every night and has Blinsky make the dolls of them (and man does that make finding the dolls an infinitely spookier moment for the players than finding the Ireena dolls), but he knows it’s his sibling, and his obsession manifests as a belief that they’re still alive, rather than lusting after them. He does it because he likes dolls, and he wants them as a sort of record of the changes the lost sibling in his dreams goes through over the years.
When he meets his sibling, he’s positively delighted - for as long as he can remember, he’s believed they’re still out there somewhere, and everyone he’s ever told about it has told him he was crazy, it was all in his head, a manifestation of the trauma of losing his family, but now here they are. And he will do
absolutely anything in his power to make sure he doesn’t lose them again. This potentially makes him a staunch ally of the PCs, and in fact I have him take Perrywimple’s place as one of the possible fated allies against Strahd in the Tarokka reading.
Finally, as I mentioned earlier, I soften Izek up quite a bit. He isn’t a cruel sadist, not really, but he has a reputation for being brutal because a.) he’s the captain of the guard in an authoritarian police state and b.) he’s huge and muscular and he has a scary demon arm, of course people are going to assume the worst of him. But in my take on him, he serves in the guard not out of a desire to have power to exploit over the weak, but out of a misguided sense of loyalty to Vallaki and to the Vallakov family in particular. They took him in when he was lost and alone, and he sees them as family. And he values family above everything else.
When everything goes down on the festival of the Blazing Sun, where Izek stands depends on his relationship with the PCs, and his sibling in particular, and how they feel about the Vallakovs. If the PCs oppose Vargas, Izek will be torn between his two families, but ultimately he will side with his sibling, as long as they’ve shown him understanding and compassion. They may even be able to get him to see that his loyalty to the Vallakovs is misguided, though part of him will always love them, he can come to understand that they were not good for Vallaki.