Getting really sick of the double-standard...
I'm a rabid Anglophile. Please don't forget that.
Actually, thinking about it, I had an instance as king of Denmark, where the king of Norway wouldn't marry my son to any of his daughters, but would accept a marriage between the same son and his 40 year old sister (I didn't have the heart, the lad was only 16...).
That seems to indicate that it is an issue of the AI valuing the daughter to much, rather than not wanting the alliance for political reasons.
I guess logically this would be based on mutual religion, culture, geographical location, approximate strength ie provinces held/manpower/gdp, dynastic prestige, de jure and de facto considerations, recent marriages (going back 2-3 generations), current marriages and alliances then mitigated by diplomacy/opinion.
It doesn't seem to be the case (surprisingly) hehe. I mean a basic system would be to go by title rank and opinion alone which is what seems to be the case. Altho dynastic prestige matters a lot as I have been unable to marry princes/princesses of smaller kingdoms to dukes in other, larger ones.
Its funny that CK2 seems to have a lot of stuff designed not to over-tax hardware requirements so a lot of complexity is streamlined in to simpler procedures. I'm pretty sure there's a lot of headroom for more stacking of conditions in a lot of this stuff.
I don't fight too many wars (for a kingdom), I'm strong, people love me, still it's impossible to let my youngest daughter marry any prince, there is like only 1 duke in the list and the rest are all counts that she can marry.