Now where i disagree is on genetic purity what even does that mean. All creatures want to adapt and evolve and get better that is they do not want purity at all the result of all those royals you mentioned was that their children where inbred , mentally ill , sickly and often then not led to the downfall of their dynasty.
Yes, if we were supposed to maintain genetical purity, the consequences wouldn't be mental illness and biological malformation. Since that is what happens with inbreed families, we already have a clue that it should be avoided at all costs. If there is an universal rule which allows you to reincarnate in the same family generation after generation, you don't have to breed with family members in order to make this work. It will just happen anyway.
All the problems start when humans try playing God's role, like the pharaohs' dinasties from the example did in ancient Egypt, and also the medieval european dinasties which married family members among themselves.