Equate the stages to real life, and you have an 18 year old with a 13 year child. Sims that the player creates are always at the very beginning of their life stage. It is possible to create a family where a young adult (i.e.Your cousin, your parents' niece or nephew, is off-limits, but that cousin's son or daughter is a-okay. This means it's entirely possible without cheats to have romantic relations with cousins, and not necessarily distant cousins either. The Sims 3 keeps track of family connections via the family tree, but if you have a massive family, it won't recognize extended relatives beyond the third generation and first cousins.
There are mods to accommodate this, however. note This was done intentionally on EA Games' part because sims don't usually live long enough to meet their great grandkids unless cheat codes are used. This means that sims can date their own great grandkids. The Sims 2 only registers three generations as relatives to each other.If you get your sim's science skill to 9 you can clone objects and sims, and these sims have no familial ties. It's possible to have a form of incest without even leaving the household if you have the University Life expansion for The Sims 3.(This troper has tried, and it works.) It probably works other ways as well ( Kissing Cousins, Parental Incest etc.) Now, it's possible for both of these children to marry each other and have kids. The child who was taken away gets adopted by another family and grows up in theirs as well. So let's say the family had another child who is healthy, and then grows up to an adult. That means that they permanently cut any family bonds the child had with them, even by blood. You see, if you don't take care of a certain child in the family, the social worker will eventually take him/her away from their parents. It may sound a bit far-fetched, but hear me out. This troper thought for a bit and calculated that it's entirely possible for incest to ensue.