I see it as a split lineage. Ayla and Kino started the line, it continued until about Zeal when everything was disrupted everything. Two lines came from this, one royal, one poser-royal. Of course poser-royal succeded to Guardia. Chrono could possibly be of the true royal, while Marle is of Guardia's pseudo-royal. Although I also believe that Marle and Schala are distant reletives whom descended from Ayla and Kino. I don't know, its weird.