I hate how Jeanne is a designated bitch as opposed to Purity Sue Rosalie. I know that some Jeanne's actions are morally questionable at best, but the thing is, we are supposed to see her as evil from the very beginning, because she resents living in poverty and doesn't want to "be like her sister" (cue the picture of smiling, wide - eyed, Tastes Like Diabetes blonde). I hoped that this is going to be subverted somehow - rebelious Jeanne would turn out to be good one, and mommy's favourite Rosalie would be the bad one. But noo. And it all went downhill from there for Jeanne, because the mangaka just had to make her into real manipulative bitch. Come on, she was a poor teenage girl. It's not really strange for poor teenage girl to be selfish and shallow. They're not suddenly monsters because of that.
Jeanne is not monstrous or evil because she wanted to escape poverty. However, it is more than "morally questionable" for her to murder innocents, manipulate, lie, and nearly kill her sister to gain power and wealth.
And let's be honest. A good part of your argument is shot down by you using the Mary Sue card against Rosalie, since the term as completely lost its meaning due to how the Fan Dumb uses it againt 99% of female characters. IIRC, there IS a "jusrisimprudence law" about 'how referring to a canon character as a Mary Sue is a sure way to actually lose the whole argument', since it relays less on canon reasons and more on "OMG I HATE THE BITCH, I'LL CALL HER SUE BECAUSE I SAY SO!"
The issue with Jeanne was less that she was evil — she was, without question — than that she was sometimes Stupid Evil in a way that didn't fit with her otherwise at least moderately intelligent villainy. So for example, when Rosalie visited her she had her beaten. Um — Why again? It was supposed to be so she wouldn't tell anyone what she knew of her (Jeanne's) past. So, she was afraid her cute, nice, Incorruptible Pure Pureness little sister who would never hurt a fly would blackmail her or try to ruin her reputation ... so in order to prevent that she gave her a really good reason to hate her and want to do so?
That was for two reasons. One, Jeanne knew Rosalie well enough that she was sure she would never act against her if not by accident (sure, in the end Rosalie made sure Oscar knew where to find her, but Jeanne never learned about her 'friendship' with Oscar). Two, Jeanne was in full Oh, Crap mode and wasn't thinking straight.
Why did Oscar consent to Rosalie's moving to the Polignacs? All right if she wants her to be able to make up her own mind, but it was rather obvious that Rosalie was acting under duress, or at the very least not thinking very straight at the time. At that, even if she'd seemed perfectly fine and coherent about it, most guardians wouldn't allow their charge to move away at a moment's notice, especially not to a Manipulative Bitch with sociopathic tendencies. So why not at least advise some caution and approach the matter something like, "Let's talk about this and wait a couple of weeks, and then you can go if you really want to?"
She also doesn't seem to make any attempt whatever to keep track of what happens to Rosalie afterward, even though she's very much aware of Polignac's notably poor record as a mother and what she did to Charlotte. Out of universe, obviously poor Rosalie was Put On The Bus, but it's very jarring considering how much Oscar seemed to care about her previously.