I know the topic is a month old, and the post I'm replying to is even older, but I saw this and had to add my two cents . . .
That would require that the software maintain a database of the grandparents for each and every villager (once your tribe gets beyond its first round of villagers). Given that it's possible to have as many as 115 villagers at a time, that's a lot of data that has to be maintained.
I don't really see this as being that difficult; it certainly seems to be a lot to track, but all you really have to do is record the parents when each child is born, which we know the game already does. Also, it would need to reserve the data until all grandchildren die off.
When a child is born, parents are A and B, so look up the parents of A, and the parents for B to determine possible variations. When a villager dies, find its grandparents, then see if those villagers line is dead down to two generations; if so, drop the data. (This means that when generation 3 dies off, generation 1 will know it; so drop the data about generation 2's parents, since generation 4 will never use it. A little confusing, I realize.) Seeing as the sum total of a villager for the purposes of genetics is a head number, it seems like a small amount of data for even a weak computer (by today's standards) to handle. In addition, the program can generate and track this database as the villagers are born; it's not like the programmers have to program a huge, 125-villager family tree into the game, complete with blanks to be filled in, and room for 250 parents and 500 grandparents.
Granted, I kind of doubt grandparents are a factor in the deviant results; such random outcomes as we see in life would be easily replicated by occasionally using a random number generator instead of a close average. However, I don't see any difficulty in the idea at all.