What can we do?

A LOT really. To change the society is to start changing yourself. See Fellow Citizens, with the same respect that you would like to be treated yourself. Change your mindset and reflect that new mindset in your dealing with the other members of the Society. If your sister or any other relative select the partner from any other region, religion, or caste give full credence and respect to the new relationship. Try to spread idea of India is being one whole and not a divided society from within. You can do a lot by spreading this idea to the people around you and that will be a big help to make India acohesive society.