I think it has to do with the royal family, and special training. Perhaps it's an innate healing Guardian gift passed down in the females of the family, that drains the chi or life force of the healer. It would explain why her mother is dead/missing, why they're so mad when she runs away, and intense mental conditioning/brainwashing in the castle would be decent motivation for her to want to leave and have a "normal" life. If she had to run away just to visit a festival, it's not outside the realm of possibility.
Even if these ideas are proven bunk, they'd still make for a far more interesting (albeit fanfic-y) Marle backstory.
It's a great question, Nang, and one worth discussing.
Aside from that, I've always assumed that the Guardian family line was an extension of the Zeal family line and that Ayla was a direct ancestor of Zeal, and that Doan was somehow related to Marle down the line as well, but that may have just been my 3rd grade 1995 mind assuming that because they're in charge, they must have other things in common. Other than Marle having that pendant, are there any other hints that any of them are related at all? If so, it may have something to do with the blood.