My dynasty looks like a telephone pole....
Fyi, it is currently possible to marry a brother to a sister in the dynasty stystem.
In my current playthrough (Altar), one of my 'grandsons' stayed with me and a 'grandaughter' swore allegiance to tarth. This was a brother sister pair, and then i promptly set up a wedding between the two, who are now both married.
Itll be intersting to see the children- will the game recognize them as two couples or as one?