To be fair it might just be because different schools suit different kids, I have a friend who has a daughter away at boarding school, begged to go and is thriving there but her sibling won’t be going because I they would hate it. Some schools are more academic, some sporty etc. If they are deliberately being treated differently then obviously that’s tit but it would be awful to stop a child going to a school that would be amazing for them just because it wouldn’t suit their sibling.
I am surprised that they don’t home ed tbh given that she is such a free spirit, going to school seems at odds with her persona. They can obviously afford tutors etc to help.