Before the patch, it's a little confusing.

After the patch, the game handles heir much better.

There are 3 groups of potential heir: sons, younger brothers and nephews. However, the game doesn't clearly tell you which one is which.

- Son: always got the throne when mature. Once, one of the son is mature, he will be put on top of the list and highline in red to indicate that he will get the throne.

- Younger brother: if the current King has no son or has no mature son, the oldest younger brother will be highline and will get the throne if the King die.

- Nephew: they are kind of back up for the other two. They won't get the throne unless none of the two types above are available.

Who will get the bride: suppose no one is married yet. The one that is highline in red will get the brides first. Then it goes down the list.

- Sons: if the current King has mature sons. The oldest one will get the birdes first. Then it will continue untill all mature sons are married.

- Brothers: if the current King has no son or no mature son or all sons are married, the younger brothers will get the bride until all are married. He will have to pass when one of the son mature.

- Nephews: they are last in the chain.

That's what I have been observing.