You need to watch out for very old kings with very old heirs. I don't think an heir gets ANY children until he is king. (At least, in my experience, they all take the throne with only brothers for heirs. I guess its a bug) . So an 87 year old king with two old sons is a risk because the sons may not have time to have children before they die themselves.
Care to explain how you can get two hiers at the same age, one ends up as thwe first hier and the other last, without the hiers getting children before they get to the throne and the first one is the new king's son and the other one is the old king's son?

I would say that Tricky Lady is correct.

Having married sons is somewhat tricky though. Usually is most princesses from one faction for the entire campaign.