I've had an experience with the incest bug, too, but in my case it first happened while I was playing the family in question.
My sim was having a house party at his residence, where he lives with his three sisters (all of them are young adults). During the party, he was in a group conversation with two of the sisters and another unrelated woman he has flirted with in the past. While still in the group conversation, he made a "flirtatious joke" directed at the unrelated woman, but as a result all three of the women, including his two sisters, gained romantic relationship with him.
I have heard of this bug before, but this is the first time I've experienced it personally in my game. That family is the only family I play in that particular save file, so it could not have happened autonomously while I was playing another household, not to mention that when I first started playing the day I first noticed it, the brother had a normal familial relationship (good friends) with both sisters and no romance bar with them whatsoever (I tend to monitor my sims' relationships very closely, and I would have noticed it).
I do not use any mods or custom content in my game. I have both expansions and a few of the packs, but I don't have "Dine Out" yet. This definitely happened since the most recent patch.
