hi! there's lots of room for more information (particularly on the question of "how prevalent"), but meanwhile, you may find these of interest

• How much incest was there really amongst european royalty?

• What prevented the unification of Europe through inter marriage of the ruling families?

• How bad a problem was inbreeding among European nobility between 1000 and 1800? Are there any other cases as bad as Charles II of Spain, or was he an extreme outlier?

• What exactly were Charles II of Spain's disabilities and how did they come about?

• Did people in medieval Europe ever link birth defects to inbreeding? If not, when was that link found?

• Were the Habsburgs aware about the dangers of inbreeding?

• Did the Hapsburgs have any clue that their marriage traditions were weakening their lineage?