You certainly want to tell me why in a recent SP English campaign, when William died and Rufus has a son that was of age (adopted actually), that adopted son was not the new heir but Henry (the youngest of William's sons) became heir?

This has nothing to do with him being adopted btw. "Normally" they get to become heir in such situations, but its peculiar that it did not happen in that case (I wasn't able to reproduce it though in another campaign). So there seems to be a random element involved.