My only question so far is... if she suffered so much in that house why didn't she just leave when she was old enough? She was picked up from the street so if she deemed living on the streets had been better than being bullied she could have gone back to that life. I doubt the brothers would go through lengths to bring her back. So she obviously found advantages to living in that house for 20 years despite the abuses. Note that I'm not condoning the abuses. I'm just asking a very valid question. She made her decision to stay for a better life, all things considered.
About the parents, they were grieving and not in a healthy way. It put additional stress and resentment on their grieving sons, who in turn acted out on the "replacement." Honestly if my parents did that, I as a child would have made life hell for that child too just so she would leave. She is mentally 27 now, and should be matured enough to understand why the children were acting that way. She does not have to tolerate their behaviors, absolutely not, but can show some understanding and compassion towards this family who adopted her, at least in her thoughts. I think the FMC is also selfish. I can try and justify that she thinks this is all a dream so she can just do whatever she wants. Hopefully we'll get some character developments for the 3 brothers and FMC, and maybe she has enough heart to try and prevent the parents' deaths. Rant over...