Uh did everyone forget the wagon incident? At that point, Aria only intended to save herself by getting with Oscar. Mielle (likely with the help of her maid) attempts to assassinate her the moment she sees her getting too close to Oscar. Remember how old they were?
Aria is common born. There's no way she'd actually get to marry Oscar and while Aria didn't know that, Mielle did. Even if Aria had his affections now, whose to say his feelings wouldn't fade later? There was literally no reason to attempt an assassination. But Mielle is at the point where she was a willing participant who calculated her way out. Had Aria actually been that young, she would have died or at the very least, had her reputation further ruined (which is everything in their society) for protesting what could have made her lose her life.
Aria was rash, impulsive, vain, and petty, but she never used her hourglass to kill Mielle. Mielle likely wouldn't have given her that courtesy.
Concerning is an adult using their maturity to corner a child. More concerning is the child willing to attempt murder because her pretty new stepsister got too close to her crush.
The young Aria didn't stand a chance, but the older Aria was able to improve herself *enough* to stand a chance. That should tell you something.
From the very beginning, Mielle had all the love, the wealth, the power, the support, and received all the benefit and credit that came with it (even when it wasn't hers, as we can see from the fur matter). Yet she still had the time to push down her originally naive and uneducated new stepsister and trap her with such a malicious scheme. Mielle is made to be evil and disliked, which is what I dislike about the series. She's too one dimensional. She and Isis completely lose their IQ at one point in the story.
What I like the most would be that Aria had to change (which she does, even if she doesn't admit it easily) and see beyond herself to survive. She had to grow up and become someone who could remember her roots and sympathize with others, which she does. To herself she's a villainess, but when you look at her actions, she's actually not bad.