Looking through the thread about Crusader Kings on a CK Reddit, I ran into the following throwaway comment:
The genocides, the murders, the marrying your grandmother and having kids……..beautiful!
I know we discussed other weird marriages in this game here (” Is marrying my own sister a good idea? “), but can the game actually be practically played in a way that you can marry your grandmother and have kids? How?
In the best possible case, this would be the progression: Your grandmother, Alice, marries Bob at the age of 16 (minimum age) and has a child, Charlie, when she turns 17 say. Charlie marries at 16 and has a child at 17, you. At this time your grandmother Alice is 34 years old. By the time you turn 16, your grandmother will have turned 50. The game makes women completely infertile at 45 (see CK2 wiki), so you would not be able to have children.
Even if the people in the chain somehow managed to have children immediately after marriage at 16 (practically impossible) that would still make your grandmother 48 when you’re first eligible to marry her, past her child bearing years.