I'm writing something for a world I'm creating, and I have this idea for a family that is made up of X number of families, all fucking one another. But I also want to keep the inbreeding to at least 3 generations separated. While also keeping the partners around the same age.

So for example, if each family was a letter, A, B, C, D, E, F, G so on. A and B having a kid would make the kid AB, So if AB fucked CD, it would make ABCD. To avoid incest they'd need to fuck with someone who had none of those letters. So EFGH. How many families would I need for A and B to be the great grandparents, before the current generation had a child with someone who has an A or a B?