Features incest, sexual and/or romantic relationships between close biological relatives, though in some instances stepfamily can also be considered.
The first video game about Incest was released on December 26, 1996.
Razzart Visual has published all these games
Historically this effectively denied political marriages, had the still present problem with higher potential of birth defects in the offspring which also reduced the viability of a family line for marriage into other families (thus a family line could die off due to inbreeding).
Note that the above mentioned birth defects can happen even without inbreeding or any other tampering (natural or not) and usually requires several generations before anything noticeable will happen. These are generally recessed traits already present in the family line. You're far more likely to get anything like this simply by drinking alcohol (or taking any other drugs) during pregnancy, even at "moderated" amounts, and that's just in one generation unlike what inbreeding requires.