You know the best revenge she could do? Actually get her sister to not be a cunt. Stem the tide of hatred before she goes over the edge.
I mean, look at it from the perspective of this rich well to do girl. Her family was invaded by a harlot and her sin baby and lets be honest, our heroine wasn't doing herself any favors. Those around little sis failed her in her upbringing to let her reach a point she's willing to plot someones death. We're supposed to hate her sister because of what she does in the future when the series started, but as I read it, and our heroine consistently bends everything to her favor, the more I realize that I really pity her sister. She's going to get the typical villainess ending, but I'd be more interested in seeing her get redeemed. It isn't until the future that the things she does is particularly malevolent. Up till now, she hasn't done anything particularly terrible, if not mean spirited because her family got encroached upon. Kids do this. Imagine the damage kids with legitimate power could do, and that's the problem here. Boxing her into a corner is only going to make her get more desperate and push forward that future you hate so much. You might not be in the same position, but is it really worth it to put HER head on the chopping block when it could probably be avoided?