What could cause a sister to believe her brother left her?

Context aside, the protagonist in my story has a sister, who after turning very sick and administered to a hospital, was said to have died. However, the sister was found to have high levels of P.P.E. (PowerPotential Energy, an energy source that triggers hidden potential within a person) and was instead kept under observation, studied, and experimented on after she had recovered. The entire time, the sister was told that the MC had left her there, possibly in order to take care of himself (they were orphans for a while). She is adamant on her belief that her brother would never do that, but I need something that would really cause a deep hatred for her brother that would cause her to actively pursue him as the antagonist for my story.

Any ideas? :/

Doubt.
No matter how much we believe in someone there is still that element of doubt deep within. If she holds in her mind that he would never leave her and will in fact come back for her then every day/week/year that passes with him not coming for her will expand that seed of doubt until finally, after perhaps some horrible event in her life perhaps a really nasty experiment, she realises that he is never coming for her and has in fact left her.
Hate ensues.
This could set up some quite emotional scenes between them in which she is all 'You never came, I waited and you never came' etc

Hope this helps

Another way with doubt is that they feed her lies about her brothers. At first she'll probably doubt them. After a few weeks, unless she's extremely strong-willed, she would probably begin doubting her brother. And if her brother wasn't around a lot, well, that would make the lies seem more likely. If her brother was about 18+ and she was about 10, then she would have faith in her brother. If her brother never seemed to pay much attention to her, it would be easy to plant the seed of hate inside her.
Or maybe the fact that her brother wasn't with her did it. If she depended on her brother for a multitude of things, such as protection, food, shelter, comfort e.t.c, and her brother didn't do it for her while she was there, she could feel as if he never let her do anything.
Either of these things could easily lead to arguments such as:

Brother: I tried to find you!
Her: They told me you forgot about me! You never got me out! You always said,'If something takes you, I will die trying to free you'. You didn't!!

And stuff like that. Hope this helps you with you're story :)