closer to step mother, the MC is the daughter of the first wife who was from a high ranked family but eloped and died in childbirth, the mother is the 2nd wife who had a child a couple days later, the father faked the birth records (which is a serious crime for their nobility) and told the mother she had twins fearing how she would treat the dead 1st wife's child

Apparently he has been kept away doing work by the "mother"'s father for several years. Who knows the truth and helped fake the birth records but also deliberately planted the seeds leading to the MC's abuse, as well there appears to be some complications with the "mother"'s birth that means she is not technically a noble (child of a commoner concubine or something like that) but was presented as one for marriage. From his reaction when he shows up at the end of the arc the father has explicitly told the "mother" not to mistreat the MC before

The MC's skill is seed production she can use magic to create the seed for any plant. Which sounds simple but is actually extremely useful, and can upset economies. Groups have withheld seeds of highly desired plants in the past to maintain effective monopolies (coffee being one example), and her ability gets around that

They test her on extinct species, and plants from myth with her being able to generate anything she can imagine, even modifying the natures of plants (she is asked to make an out of season seed for a duke that is needed to cure a fatal disease his grand daughter contracted but the seed contains a toxin that causes other lasting issues, though she would live, the MC makes the seed without the toxin and full medicinal effect)

The first day she gets up early and starts cleaning out of habit, which causes the guards to go on alert thinking something is wrong, as well she is severely malnourished which caused her to look years younger as well as having major developmental defects (low mana pool and permanently stunted growth) that they end up using healing magic on

the first on a public holiday, they run into each other in some gardens then the sister "orders" the MC to change her cloths and goes to beat the MC only to be stopped by some knights. She tries to tell them to release her and that she was trying to "discipline" the MC and seems to get annoyed when they do not listen.

The second time the MC is outside in a field and notices a group stepping out for a session of the skill control training, including her sister, and with escorts goes to watch. The sister outright ties to kill the MC by launching a maxed fireball at her (seems to be a combination of lack of control of her emotions, a sense pride and that she should be superior, and jealousy over the MC getting better treatment) when she was supposed to be making a small controlled fireball. (Those who fail control training have their powers sealed, and can lose their status as nobels)

after using her skill to save the life of a duke's, who is a relative of the king, grand daughter they give her a permanent place at the research institute as a reward and she is adopted by a relative of her biological mother (the faked birth records meant they thought there was no child they needed to look in on, but an encounter with one member has him start to look in to things because the MC strongly resembles her biological mother). Her family loses their rank and pierage, due to several issues including faking records and fraud, with the sisters power being sealed