It's cool that Mia is actually on Philia's side here, but I don't understand how Mia can b...

But it isnt her whole country; its largely her parents and the prince and aristocracy. Because we see in the first chapter, working folk from her homeland still spoke to her and engaged with her normally with some gratitude (though perhaps not as much as deserved.)
In regard to Mia, I would say its because its normal. It's difficult to see something as wrong when it is so normalised, and her sister has been treated that way since...forever. It also may not have been obvious, their parents likely wouldnt have been overtly abusive and horrible when Mia was present, likely instead focusing on fawning over their beloved youngest.

"The whole country" was a bit dramatic ngl, I agree with you. But the hate for Philia from these people feels artificial in the end. It's excessive to the point where it makes you question Mia's passive attitude. I don't want to sus her out either and I don't think there's anything there to build conspiracies on, it's a mistake on the writer's part for missing all this.
Anyway, I hope this is gonna be explained in a bit more detail. We shall see-

And I don't think Mia would just accept that it's normal for Philia to be mildly to severly disrespected. Especially because Mia must've seen the contrast in how they're both treated, yk what I mean? Perhaps it's more so a thing where she couldn't do much of anything, at least I hope it is.

I made up my own head-cannon recently: Philia's holy presence is so shiny and pure that it starts to feel unnatural and disgusting to humans who have largely dark 'sinful' souls aka selfish assholes like the prince, parents etc.
Mia as a saintess herself would be unaffected but wouldn't know where to start in altering that kind of effect either way. (Perhaps her own powers who're the average amount of purifying temporarily removed Philia's "purging effect" which made certain assholes adore Mia and compare both sisters the way they did constantly).
God, I need sleep-

It seems to me that everyone was so focused on Mia when she was around that they never brought up her sister when she was around, with the parents and the prince seemed to be very manipulative from the get-go; they probably only showed Mia their good side, only actively degrading and mistreating Philia when Mia wasn't around, based on how their family and country treated saintesses (and women) it wouldn't surprise me if Mia was also busy all day just not to the same extent, mistreatment, or pressure as Philia, in my personal opinion it seems like a scapegoat and golden child situation mixed with Philia being so powerful that she inadvertently emasculated the prince, nobles, and knights catching their ire when she was only trying to follow her duties and make up for what she was told she lacked (so basically the whole situation wouldn't be as bad if she had decent human beings as parents)

It seems to me that everyone was so focused on Mia when she was around that they never brought up her sister when she was around, with the parents and the prince seemed to be very manipulative from the get-go; they probably only showed Mia their good side, only actively degrading and mistreating Philia when Mia wasn't around, based on how their family and country treated saintesses (and women) it wouldn't surprise me if Mia was also busy all day just not to the same extent, mistreatment, or pressure as Philia, in my personal opinion it seems like a scapegoat and golden child situation mixed with Philia being so powerful that she inadvertently emasculated the prince, nobles, and knights catching their ire when she was only trying to follow her duties and make up for what she was told she lacked (so basically the whole situation wouldn't be as bad if she had decent human beings as parents )