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The reasons for her behavior may be different. Envy is one. Other possibilities may be that she is projecting her own shortcomings onto you. That way, she takes the attention away from herself and places it onto another person.
She may also be doing this not because she is jealous of you, but of her brother. She may have an inferiority complex. Why she is doing it may not be even clear to her. Behavior serves a purpose even if it is unhealthy. It is serving her one way or another. She may be getting attention. Even negative attention is preferred to lack of attention all together.
What would be helpful is not to fight with her. Try not to take it personal (even she's personally trying to attack you) Instead, remain kind to her. She must be in turmoil if all she is experiencing are negative and hateful feelings. What kind of life is that for anyone? Try to ignore her words and behaviors and see the person behind it.