I feel like it would be more apt to title this:
Recurring themes and tropes and common faults in incest-themed games
A lot of the options sort of boil down to lack of originality, which is often the case with lots of games, but noticeable in games with incest.
Some of these I can't really fault the developers for. For example, "Same old family unit: Mom, Older Sister,...". However, it would be nice if developers realized there's more related members in a family than those directly next to you in your family tree. We've got cousins, aunts, nieces, nephews, etc. I might start an argument by saying this, but you've even got step relatives to choose from as well!
There is no limit to taboo!