What counts as incest?

I've been breeding my Vault Dwellers together pretty indiscriminately, and as such I really have no idea what their family trees look like. I don't have much to go on other than their last name, which is a imperfect metric at best. Not infrequently I'll put two dwellers into living quarters only to find out that they're considered related. How does the game determine whether or not two dwellers are related?

This question is largely academic, since I haven't been keeping records of who had reproduced with who, but I'm still curious.

Solution 1:

Game records within each dweller who are his/her parents, and their respective parents. When you try to pair up two dwellers, it'll check if they're related this way. It'll check too if they don't have the same parents. Otherwise you can breed your dwellers as you like. You can pair up a dweller with his/her grandchild's wife/husband for example.

Solution 2:

As far as I've been able to determine through my own experience, and what I've read else where, if the people are within three generations of each other it counts as "family." The only exception to this rule is that aunts and uncles can get freaky with their nieces and nephews assuming there is no direct relation somewhere up the family line.