speaking for myself, it is actually a kink of mine, in that I like the notion of being involved with 2 women who are blood relation to one another, but not to me.
I also acknowledge that it is purely a fantasy (I would never do anything like that with my actual sisters/mother/cousins).. In fact, I even once had the chance to go home with a mother and daughter couple who would have been up for it.. but I refused because of a bunch of practical and ethical reasons.. the real world implications,, fallout and consequences would have been just too damaging for me to want to be cause of/involved in it.
o I do have the fantasy but that's where it needs to stay, and will always stay. Of course that means that incest in games is not at all unappealing to me.
That said, yes, I was rather surprised to find it to be such a prevalent theme in these games. I thought most people would be freaked/grossed out by it... but supply and demand seems to indicate that this is not the case.