I'm figuring your sister was 12 when you were diagnosed with cancer, so she was also very young and she's obviously not gotten over it. It's a shame your parents weren't able to deal with the jealously she must have felt at all the attention you were getting. It's a very immature reaction to your situation, but at 12 you can't blame her.
But you can blame her for holding on to it all these years. She needs to let it go, it's not your fault.
How long were you in treatment for? And how does your mother treat you now?