"Your aunt is the sister of one of your parents."
Translation:Via onklino estas la fratino de unu el viaj gepatroj.
Well, the usage of de is unfortunately much more broad and inconsistent than that. You'd think Esperanto would've been given a different preposition for each different meaning of "of", but alas even Esperanto is based off of some mutual expectations here and there. Calling it a "neutral language" is an overstatement; it's full of flaws and idioms that simply result from its being based off of solely European languages.