The way (some) people here manage to jump headfirst into stuffy Victorian moralism without even noticing never ceases to amaze, disappoint and amuse me in equal measure.
Morals were never a factor as far as I'm concerned.
You sure spend a lot of words condemning her "slutty" habits for that. Nevermind now parroting about verbatim the usual checklist Moral Guardian types tend to level against nigh any sexual behaviour outside whatever particular narrow stripe they find to be acceptable; I'm slightly disappointed at the lack of "think of the children!" there to be honest.
Now Miyuki's conduct towards Asuka can certainly disagreed with already on basis of the latter's age and not at all helped by the former's position as a surrogate big sister of sorts, but this and that are two quite separate matters.
Aside from manipulating and hurting people of course, like Miyuki does.
I suppose the former would be guilty as charged already by the default that "seduction" certainly falls under the rubric of interpersonal manipulation; the latter would be rather debatable and require citing what you think implies she's intentionally doing so just for starters.