My son is exactly the same. He resents his sister so much; he is 16 and she is 20 and they got along pretty well when they were younger but for the last several years he complains that her life is perfect and she has everything easy and why cant his life be like that ? So I know its jealousy and also in their verbal fights, she always wins with her come backs and all and he cannot even respond so the resentment grew over time. Now he does not want me to mention her. When we have guests or family over and people talk to her, he does nt like it. I don't know either how to handle that. I have conveyed this to his therapist though...
Sorry I could nt help but this is the situation with us too
All the best