Syl says in one part of the game that you "shall pay for my brother's death". Is she calling Thadon brother in the sense that they are both dukes or does she mean they are literally brother and sister?

If so, then they really like the incist.

I'm sure they weren't biological siblings. Like Hellmouth said. She just meant "brother" in the sense of "peer", "fellow".

C'mon. Twincist is not only more kinky interesting, it's more in fitting with Sheo's domain.

I was implying "I have no [censored] idea, but..."

Yeah, I like the incist idea. Fitting for the Madhouse.

Well okay then. Wincist it is.
And Hellmouth, I know what you were saying, I just assumed that it was a point against them being actual siblings lol.

I suppose it could be either, since there doesn't really seem to be any in-game evidence.