Isabelle doesn't hate Clary. She just doesn't want her anywhere near her brothers, or her friends (the few she has, anyway), and nowhere even close to Jace. And it's not Clary's fault, she knows, that Jace feels the way he does. Not her fault that she's sweet and kind and understanding and everything Jace needs, but can't have. That makes it worse, she thinks, because if Jace&Clary even had a chance of being, everything would be magically fixed. And it's not fair that the opposite -just plain old Jace and plain old Clary and absolutely no 'and' about it- hurts ten times worse.
Her heart breaks for her brother every time she sees him staring at the red headed girl with a lost expression on his face. He doesn't deserve that kind of pain, to have something he desperately wants (loves, but she isn't ready to say those words out loud yet, not even in her mind) dangled in front of him.
And neither did she, come to think of it. Because, once Isabelle pushes past petty jealous and protective familial instincts, Clary is actually a nice person. The kind of person she'd want as a friend, if things were different. She can imagine them gossiping and training and doing each other's nails and killing demons and- damnit, she's crying because now that she's thinking about it, Clary could be the sister she's always wanted.
And, really, it's not fair that they all get to lose her, too.
But Isabelle knows that leaving her behind is the best thing.
Because no matter how many what-ifs and what-could-bes she thinks of, Jace is still her brother. And his heart is still broken, and this girl is still the one that caused it.
And, really, it's just not fair.