The 2 people who made reiko's life the hardest were the housekeeper(can't remember her name), who essentially ruined the grandmother and reiko's family and drove the grandmother into a corner due to having a weird obsession with the grandmother, she hated reiko and if she could have gotten away with it, would probably have killed her, even after the grandmother's death she still when she can, makes reiko's life hard, this is only a small part of the many people's lives she ruined, she's primarily responsible for her stepmother's infertility too as a example.
The other is reiko's father, remember how i said the grandmother isn't as bad as you might believe at first?
The grandmother had investments, stocks etc she had saved back and set aside so that when she died reiko would have been taken care of financially as while she was prideful, she still loved her, the father tricked reiko into signing away her inheritance to him (which she didn't know she had, thank the housekeeper and dad for that), he's also the one who destroyed her family's fortune out of "revenge" for forcing him to give up the love of his life (which was again, the housekeeper's doing, not the family's) and is not remorseful for it, honestly the only reason why he probably doesn't get too much of a comeuppance is due to him being married to the stepmother.
The stepmother, who is also reiko's landlord (and i believe before marrying the shit dad, not exactly well off either), LOVES reiko as her daughter and while reiko has to more or less push her into it, it's implied that for both her and reiko that the reason she does later in the story marry the shit father is because it will officially make her reiko's mom and give her access to the shit father's fortune to help reiko out as well as ensure reiko get's her inheritance at some point, since she is NOT happy with the father over his treatment of reiko either (i believe this was one of if not THE major reasons she wasn't already married to him), the stepmother sincerely loves reiko and views her as her only daugher and the feeling is mutual.
The stepmother got a particularly heartbreaking act done on her by the housekeeper, the housekeeper's meddling is what caused the father and stepmother to break up initially, but she's also responsible for the more or less forced abortion the stepmother had to get, which made her infertile, yeah the "nice" housekeeper is one of the biggest villains i've seen in this kind of manga.
Finally while the siblings are extremely close friends with reiko (with the sister being reiko's BFF throughout the story), the brother is not the ML, the ML i think hasn't been shown yet, he's the student council president and claims he's "poor" like reiko (kind of a half truth, he is poor, i not sure 100% as i might be getting a story mixed up but i believe his family is very wealthy and he wants to make his place in the world by his own hand, not by a family handout, i believe he and the brother become good friends, though love rivals for a good part of the story (the brother however, does get his own happy ending), the ml is also usually the first to pull the idiot who calls reiko a villainesss away.
I don't know the full ending but i do know they get married at the end and reiko becomes a world renown pianist like her grandmother was.
Housekeeper i don't know what happens to her other than after her grandson finds out the truth he hates her (he starts out bullying reiko in part by of course, his grandmother's manipulation, but also little miss dumbass and the so called heroine who's really the more schoolike villainess claiming how terrible she is) and wants nothing to do with her.