Eirin's a lunarian, who generally don't age. Even youkai are stated to age, I believe (I forget where) but Lunarians generally don't due to lacking impurities (though they do have a teeeeny bit of impurity so maybe they age a liiiittle).
Eirin may (as well as may not) be from Earth, considering that (from what I've heard) in the legend of Tsukuyomi, he brought some people who he'd considered to be pure, to the moon along with himself, and Eirin was (stated to be) possibly around the Lunar Capital's foundation. But there's some mentions to Eirin's family line as well, saying that her family line is very intelligent, herself being even more than the average.
My theory (unrelated, though), since Eirin's name may actually be something else,
it's possible be the Omohikane goddess in disguise (Injustice? Gods among us?)
(note, though, that I'm explicitely saying that it's a possibility, not that it's a fact, as there are other possibilities as well, for a start)
The thing about Maribel's relatives is we KNOW the gist of her family line (well, at least, we know what she knows, I admit). Maribel specifically says that her ancestors have sought to be spirited away to another world and always had an interest in the occult.
maybe they stopped using their powers at some point (like some among the tanuki have disguised themselves as humans, and forgot that they're not humans, but then they lost the ability to untransform, due to lack of usage of their powers), and the later generations wish they could know what it was like to have access to such abilities and knowledge.
#kappa
random thought: I wonder what would Yukari be like, if ZUN hasn't watched Spirited Away (or the movie itself didn't exist). Not that I'm willing to put effort into calculating that myself, since there's just too many possibilities to analyze.
Which means that she has grandparents, great grandparents, great great grandparents, etc, all the way back to ancient times.
she doesn't need to be Yukari's direct descendant, if Yukari's abilities (or some of them) are partially from her family line.
(and Maribel is... well, basically repeating or attempting to repeat what Yukari already did, which is kinda... I dunno, lame if you ask me)
but that would imply Yukari's existance (and role) is focused into being Maribel's goal, which may not be the case (partially serving as Reimu's youkai partner/counterpart/foil, or just being born out of ZUN's inspiration -- like other characters -- rather than being made to fulfill a role -- for example, like Aya's case). The part where Maribel reaches the moon like Yukari did can be interpreted as a nice nostalgic generation xerox (rather than lame), as much as it can mean that it's how the to-become "Yukari" learned that she was capable of doing that trick.
some thoughts:
- the release dates for PCB's full version and Changeability of Strange Dream are quite close, it's possible that Yukari and Maribel are made to complement each other, but not necessarily be one and the same. It's also possible that Maribel was an afterthought, or someone who serves the purpose of story telling, together with Renko. Maybe Maribel wasn't meant to cause events that interfere with Gensokyo's timeline (serving as a cameo, a wandering passenger), like Yukari did (with sending youkai to the moon, for example).
- Maribel (possibly) met Mokou (when the latter already had her ability to control fire), so if Yukari's powers are explained as being old, there are quite some other peers who should be equally or more powerful (lol pwrlvls) such as Miko, Mokou and Tewi (and Hata). Kaguya and Eirin are said to be really powerful (contrary to what the spellcard system would make people think), Eirin being even more than Kaguya, so I haven't included them.
- if the maid Maribel met in a mansion was really Sakuya, before the mansion moved into Gensokyo, one could wonder who were the mansion's previous owners.. and if Sakuya has forgotten Yukari, if we go by the Maribelkari theory
- Sakuya's also another of those who uses an alias (like Eirin).
- Shinki can be interpreted as Satan, but she does seem to be inspired by ESP Ra De's final boss, mrs. Gara Ono (though both ESP Ra De -- and Armed Police Batrider -- were released in the same year, 1998, some patterns from APB (and Dodonpachi as well, from 1997) seem to have inspired some attacks in MS.