This achievement is perhaps easier as a Zoroastrian or as a Reformed Pagan with Divine Marriage, as there are no restrictions to marrying close relatives.
The Inbred trait appears when a character's parents share common ancestors, such as the same mother or grandmother; the more common ancestors they share, the higher the likelihood of the Inbred trait being recieved.
If you are not Zoroastrian, or otherwise not able to simply marry close relatives (e.g. father and daughter, siblings), then you could just settle for marrying 1st cousins and similar relatives together, though the odds of the Inbred trait being passed on reduces, as the number of common ancestors reduces