So the little brother hates his sister, or developed a kink and he loves to see her suffering, could have started when she tried to defend him and got her ass beaten senseless.
Her parents are pure human trash, one thing is to start neglecting her, another is to forbid her from even be around her brother and hurting her phisycally, both bad, and they gone from bad to worse.
She believes her little brother to have no sin and their parents are the problem, unfortunately all 3 are rotten.
And of all the luck her helper is MC, a literal functional retard that can see the future but can't formulate a normal dialogue or come up with even a simplel excuse or act in asemi-convincing way.
Most likely her lil bro wants to hurt her best friend to see her suffer from losing someone close, the hate is because when the 2 girls are close they are happy.
The alternative is that he wants to monopolize her sister or being her only pillar, so, twisted yandere love.