Incest is best?

Best post header ever.  Good question though...  It would make sense to allow it, but then maybe apply some negatives.  I don't know there are pros and cons for allowing it and not allowing it.  I would assume they won't allow it.

Depends of game rating? If they complain for nipples or little drops of blood...

Incest can be included as long as those kids have a significant change of being born with some major vice, be it insanity or a crippled body. They should also not survive past infancy as often as other babies. 

I have to admit that the idea of Channelers breeding is amusing. But, to keep it realistic, isn't it necessary to add a big random factor that renders the process pointless except if applied on many generations?

But more seriously, I'm not sure that in-breeding would pass the 'politically correct' gate, but I agree it would be nice if there was some way to keep away from all the marriage/alliance/dowry stuff if the player chooses to, without being too much at a disadvantage.

I kind of think that medieval total war handled it best, with a major penalty trait for the immediate descendants that lessened over generations, but I think it should take a few generations to build up enough to be debilitating. I think it'd be hilariously twisted to let you marry off children with inheritable negative traits as a sort of weapon. It's amazing how crippling a few genetic disorders were, such as Mary Queen of Scots' case of porphyria screwing up endless royalty to the bursts of madness in the Spanish royal family, to the crippling effects on later rulers, especially Charles II of Spain. Oh, and whoever gave all the royal families hemophilia. It'd be very interesting to see that and also trying to keep negatives out of your family.

The concept of incest doesn't really phase me one way (or the other :P).  I'd rather it be a robust system, and if it happened to allow incest, that's fine.  I'd hate for the cool concept of royal families to be stifled just to be PC.

As for the bonus/holding back, not marrying your two sibblings to two other nations is really going to hurt you politically.  I'm not sure there is enough to gain to bother addressing it.

That's a fantastic idea, but wouldn't it be a bit difficult to keep track of potential congenital defects?
I mean, keeping track of who to marry with who and when sounds already like an ordeal considering the full picture.

there is a difference between holding true to blue blood marraige and incest.... perhaps a fine line, but still a line.

Incest is wincest!

You do realize that it takes generations for inbreeding to become significant?  Yes, if there are already any recessive issues it vastly increases the odds of them showing up, but the overall degredation of the offspring takes time for it to build.

I'd also like to point out that there's more inbreeding today (closer) than there was in the mideval era.  Cousin marriages that today would be allowed would be illegal back then.  Although of course these rules were sometimes broken, as is life.

Wha?  Many mideval royal families practically ONLY married cousins.    On top of that, Mideval communities were generally so un-diverse that marrage between any members of the same community was looking at gene pool that is more shallow than almost any set of modern American '1st cousins'.  Let alone second or third cousins.

The offspring of your brother/sister marriage has inherited Awesome Godly magical power and can lay waste to all the land. Unfortunately he/she has 6 fingers on his right hand and because of this has a 94% chance of spell failure

Incest thing was hilarious in the first Medieval Total War. The generals got some risky traits. Some of those kind of traits were random too, like those that involved an uncle or nephew/niece. In that the inquisitors could use that as evidence of heresy.

Incest does not take generations to produce debiliating effects. It just often takes generations for NEW debiliations to appear.

Any recessive trait carried in your family (sickle cell anemia, hemophilia, color blindness female baldness, black hair, etc) has a very high rate of suddenly showing up. These traits are in fact often worst in the first few generations as "natural selection" (whether natural or not) may help take care of some of it even though it will always be a recurring problem until the genepool is broadened.

Wrong. I have an aunt and uncle (both deceased) who were also first cousins. 4 children, 1 normal, 1 mentally handicapped and 2 blind from birth.

Not good odds and not generations of inbreeding. I've done genealogy on that family line back to the mid-1700s and there was no history of intermarriage.

"Incest was therefore prohibited by canon law so strict and "disadvantageous for most of the population" (Archibald, "Medieval Incest Law" 11) that there was constant tension between the Church and people: "The church was engaged in a continuous (and losing) battle to implement them" (Archibald, "Incest in Medieval Literature" 7). "

Ok, my bad, it was illegal (And more so than today), people just did it anyway.

In any case, I think the question should not be what fits reality, as the game's reality is pliable enough to make just about anything work (preasence of the channeler's hereditary magical ability combats deformity?), and the question should be more along the lines of what would be better from a strategic POV...... my money's on less chance of deformed/wierd/superhuman offspring because hyperpoweful RNGs always leave me feeling just a bit cheated.....