Honestly disregarding how weird it might be for an adopted brother or sister to pursue a romantic relationship (the only reason this isn't as weird as it could be is because the MC is mentally old, imagine if she was the young girl she was in her past life, that would be... problematic) the issue I have is honestly with the parents of the family. They are horrible. Like who thinks: "Oh my daughter just died and her twin is grieving, let me pick up a girl from the slums that looks like her and replace my dead daughter with her." Like wow, not to mention in the first few chapters the childrens father hit his son? That is not how you parent, I get it, they almost killed her, but they are children, they don't know what they are doing and not to mention, they are grieving their sister. How do you think you would feel if your parents replaced your dead sister with a knock-off version. I would feel horrible and maybe a bit resentful. May not be enough to excuse their actions but i would say that it provides a bit of background. Another note is that this whole resentment between siblings wouldn't have really happened if the parents saw this and took care of the situation, can they really not see the strained relationship? That would take a lot of work. This is borderline neglectful. One last thing, is that I feel like the MC is weirdly superior, she doesn't treat her siblings like siblings but like kids and humors them. I get it, she's an adult but the least she could do is be sympathetic but instead she thinks she's above them (though that could be a weird way of her dealing with her trauma -- which honestly, if she's been abused her whole life, it really doesn't show, it feels like an accessory to her whole 'second life' spiel.) This isn't really that different from other second chance novels though, I guess it just kind of triggered me because I have a twin as well and would hate if that happened to me therefore making sympathize with the brothers more than the MC.
I might be getting a few details wrong here and there but you get the jist.