How to convince girl's parents for inter caste marriage (BC and Reddy)

Hi. I'm Arun and I'm 23. I'm in love with a girl of 24. The main problem is we are not belong to same caste. I'm from BC and she's reddy. We love each other very much. We can't live without each other. As I'm very young I'm unable to talk about my this at my home, I'm waiting for my sister's marriage to be completed, after that I can tell to my parents. But I know that my parents will agree. I can tell that with 100% confidence. But the problem is from girl's side. One year back she told about it to her elder sister and her mother. They didn't agree because they are so much worried about what society will talk. Her sister is unable to agree because her husband don't like inter caste marriages, he's very strict about it. But the miracle is her husband once helped his friend for his love marriage but that's not an intercaste one. He has no problem with love marriage but all he cares is about caste. So my girl friend has broken down completely. She's very much attached to her family which I can also understand. We don't want to elope and get marriaged. We only marry with both our parents blessings. This is about hey sister and mother. After 6 months of this incident once her sister told to her husband that one boy is loving her sister but she has no feelings about him. So her husband called her father and told the same to him. But later her sister said our real story only to her father but not to her husband. Her father got angry and scolded my girl friend and said this inter caste won't work in out caste. Now my girl friend got depressed. Her parents are looking for matches in their caste. So please help us what should we do. Her elder sister worrying because if we get marry then her marriage life can get effected as her husband can't listen to anyone. Please help us. We can't live without each other. We are seeing hell. Little good thing is that once by seeing my girl friend crying so much her mother once told to her elder sister in phone that she (my girl friend) will be happy, what happens if we agree she can live happy na! But her elder sister didn't convince. With this we can say that her mother is ready but she alone can't do anything as no one listens to her mother at home. Amy girl friends father can't go against his son-in-law (elder sister's husband) as he respects him which is usual. Please help us out. Thanks in advance