Inter Cast Marriage Problem Help Needed.?

I am a Hindu Naidu cast guy and my girl is Rajaka cast from Andhra Pradesh. We are in love from past 4 years. We love, understand and respect each other Very well. We are software Engineers and are earning hand full of salary which makes it easy to live in a metro. Now the problem is we both have younger sister's in our family. In our family it's my sister who has to get married first and similarly she is the middle one, with her younger sister yet to be married after her. Now that her elder sister is married a year back now their parents are trying to get her marriage done. What we have done is Informed all our parents about our love, thinking that we are educated and working in MNC's.But things went wrong and these are people are now asking us not to spoil their reputation in the society by going for Inter cast marriage. They are also saying that our marriage would effect both of our younger sisters marriages. They are more concerned about society than our love. we are educated and well settled in career. We have tried enough till now and our parents are not all willing to consider our opinions. We thought of going for an edge and decided to marry first for our security and then try pleading them for marriage approval. We thought of telling that we got married only after trying our best to get them approved for our marriage. Please let us know whether our marriage would really impact a lot on our sisters? Even if we are keeping all our effort what is that we are still missing to make our parents get approved for our marriage. Does it really be a problem for our sisters if both us get our marriage done by our parents approval? Should we really sacrifice our love only for this Cast and Social Inequality present in society? We are Educated, Software Engineers. What is that thing Which is missing in us, is making us not able to satisfy our parents?Please let us know your valuable suggestions as one of your honest suggestions would really help in getting us ahead of our life.