unknown family (five generations) > miyadeguchi (two generations) > hakurei (final six gens to this day)
they, by a certain definition, have each served the hakurei shrine, with the next clan starting off as loyal students just before the previous folks are completely done
mizuchi did everything just plain perfectly and looked down upon the entire hakurei clan, back when she was alive and it used to be the "miyadeguchi shrine". allowed the power and lofty status to get to her head, you see. unfortunately, one day, mizuchi never came back, so the hakurei took it upon themselves to pick up the slack. fast forward to when reimu's in her early teens, learning the ropes on being a shrine maiden.
mizuchi intervenes while reimu's actual moms (one of whom remarried a long time after losing her husband/reimu's dad to a natural disease) hold a festival just to torture all the hakurei and reshape gensokyo into her own kingdom
it all descends into fucking CHAOS
amidst the now smog-covered skies, reimu fights like heck but mizuchi zaps the girl's worst fear into reality - not to toy with her or drive her mad, but to atomize her and then do the same to all the humans and youkai who've been friendly with each other
after mizuchi's intervention, reimu's left buried under rubble, comatose as her great-great-great grand aunt breaks her mommies down and sadistically one-shots them, before wiping out so many innocent youkai that they practically lose their minds. only an old turtle, amidst the destruction, comes along undetected and pulverizes some of the debris to snag the bloodied, beaten girl and drag her away from further harm. eventually mended by magic, the 13-or-14-year-old reimu snaps awake in a frenzy screaming for her parents.
genjii calms her down, then reimu explains all she remembers was going in for the kill against great-aunt mizuchi. genjii doesn't remember it that way: all he saw was mizuchi legitimately defeating the yakumos, kasen, and okina. all he could even do was dig reimu out of the rubble. he points out that her heart seemed to have stopped and he revived her several minutes before she bit the dust. reimu thinks it was her own fault and lets out a deafening...something constituting as a yell as she grieves.
the new hakurei miko, after three years of so-painful-it's-ridiculous training only interspersed by odd jobs to get supplies for her new "gramps", notices something is sus with the hakurei shrine. from there, soew plays out, except reimu mistakes mima for mizuchi at first. this doesn't earn her any favors with mima (whom i headcanon as being similar to the gashadokuro) or her most loyal student, marisa. however, by the end of pc-98, reimu gets to know both of them better now that she and marisa are young adults. and the rest is history.