Besides, for all we know, the Guardia line runs through Kino and that any female suitor of his would've sufficed.
I am willing to bet that is actually the case. Otherwise there would be no other possible way to explain why the Guardia Kingdom would still exist without Ayla being there in the first place to start the royal line. Besides, in the moonlight parade ending Ayla hits Kino and calls him a 'dummie' when he mentions something about her being 'very strong' and then says that they should go back to their time. I never understood why she said that to him in the first place, being that it's such a random thing to say.
One way to interpret this is that she said it because she actually knew that she was not a distant relative of hers but that Kino in contrast actually is? Maybe she knew that he was cheating on her and that's the reason he mentions how emotionally 'strong' she is and in the end they find a way to resolve this and they still end up married (as seen in the psx ending)? Or maybe perhaps the men at that time were allowed to marry more than one, and Kino actually had a child with another woman before he met Ayla?
In any case, I disregard completely the possibilty that Ayla is Marle's 'distant ancestor'. Plus, when Lucca introduces them after bringring them before the King and they each say something about what Crono has helped them accomplish, only Kino is introduced as her 'kinsmen' excluding Ayla explicitly form the prospect of being related to her.