Mr. brami, the thing is caste, religion, and status matter in our country rather than age. So my advice will to send some mediator to her father to know his mood & reaction regarding your age and you, and then you go thereafter tell all the facts and feelings and because he is a father every father will see financial security first for his daughter, what sister says is of no value-nothing when you are right on ur path. Thirdly you have to have a strong commitment from the girl in any worst situation comes in your way she will be there for you (legally). Being an Advocate i will advice you to do Registered Court Marriage to remain safe in all legal aspect for life.