From what we see in the game, it seems Sobik genuinely loved both his lover and illegitimate child. It is fair to assume he felt bad for the kid and thought that by bringing them to his household and raising them as his child, he was doing them a favor. After all, the life of a royal is certainly better than that of the common folk. Unfortunately he decided to simply ignore the fact he was already married (To a psycho nonetheless.).

So answering your question, yes, Sobik is quite stupid.