I dont think Mira is a terrible person, but her and the other older sisters bullying seems to be a large part of where Sarah's body issues stem from.
We get one example of their bullying in the game so far, and its fair to say it affects Sarah to her core
Sarah clearly has walls up that cause her to act generally rude to people, maybe save for lucy, and yet one singular interaction with her older sisters causes her much strife. To me, this would indicate that this "little boy" insult is one they have been using for some time. Not to mention they are her sisters, her insults were playground level, they went straight for the throat.
That isn't to say that Sarah is a good person and Mira isn't. Sarah writing "slut" on Victoria's face, while knowing its a moniker Victoria doesn't like. She is effectively doing the same thing as her sisters, just on a smaller scale.
I certainly dont doubt the pressure aspect, and that definitely excuses SOME of Mira's actions, for instance the conversation she has with the Mc in the most recent update:
This conversation shows the pressure she is under, and how the founding family teachings have affected her. She was taught to see other people her age as a threat, to always think about power dynamics and manipulations. Her thinking that her sister was the one who set the whole thing up isn't a normal thing that a normal person would think, it shows how her mind has been altered by her higher standings in the family.
I think the important thing to remember is that due to how the hierarchies of the families work, Sarah was not a threat to Mira at all for her entire life, Mira is an older sister of one of the earlier wives, and Sarah is the youngest of the 4th wife. So what would Mira stand to gain by bullying Sarah? Sarah was effectively a non player in the founding family crap until the MC arrived, Mira's actions towards her cannot be explained or excused by founding family pressures.
I think Mira could be redeemed, but in this VN? probably not, it doesn't seem like that's the angle Cosy is going for (but my predictions have been wrong countless times for this VN so who knows).