Quote Originally Posted by Ironside
Piety: Crusades and jihads gives one piety each. And as you can easly lunch multiple jihads, muslims can easily get maxed piety. There's a bug with that though. Lunching 10+ jihads (not sure of the exact number when this will happen, 9 is safe though) at the same target causes random piety (it probably gets higher than 10 and spills over somehow). One inquisitor line gives piety at the cost of dread and placing hiers in a high zeal province gives the zealot line. Remind you, it's piety bonus disappears at higher levels of the trait, so remove them. If you're orthodox or plays VI then you can only hope for luck to increase it, but there it doesn't matter as much.
Slight correction: generals that succesfull survive an inquisition trial will develop the "Born again" line of virtues that give increasing piety bonuses (i.e. it doesn't disappear), however they also stand a high risk of becomming a secret heretic or atheist. The next inquisition trail (provided they don't get burned) will usually reveal this vice, thus giving him a massive piety penalty. Use with caution.

Quote Originally Posted by Deus ret.
in addition I had the clear impression that heirs to a king married to a foreign lady get a slight to average boost to their stats, often making them more balanced -- on a rather high level. that's just a subjective assumption, though.
Like Ironside said, dynastic incest will not result in inbreeding vices, so I don't think this is true. Possibly you are just seeing the effect of high influence.