All family trees do seem to always branch out in a specific way. All starting characters in my campaigns always seem to have the same children in every campaign. The same applies to their children and their subsequent offspring. They also often seem to develop similarly in every campaign also.

I'm not too sure, but there probably is something hardcoded which makes this happen - its the same in modifications also. It may be the order the system looks in the name database, or it may be based on a different more obscure system.