Ok so hear me out, crackpot theory time.
Cathay theme is Yin and Yang, opposite duality. Their starting LL reflect if perfectly. One is a white-haired woman that turns into a black dragon with an emphasis on air powers (storms) and the other is a black-haired man that turns into a white dragon with an emphasis on earth powers (iron).
It follows that the Dragon Emperor cannot have a third children, since it would totally throw away this core design of the race. Hence, every other Cathayan LL will be something new !
I believe in Slaanesh supremacy