Be honest with her. She had already told you that she does not to go to India (not sure if this is definite or for the time being) It is also unclear as to how long you've been dating. Regardless, you would want to tell her about your family obligation. Surely she must of at least known that you have to go back at some point.
Try to talk to your parents if you feel that sharing this info with them would be helpful to you. Do not expect them to understand or change their minds.
Ask your girlfriend if she would be willing to keep in touch (mail, internet/skype, etc) She may decide to come and visit at least at some point in the future. If not, at least you've given her options.
You would want to talk to her some place private (where she can focus on what you are saying and also if she gets emotional, to be able to express her feelings)
You can go over a mini summary of your relationship (how it had made you fee, what your hopes were/are for it, ask her what she thinks and feels about it as well)
Make sure that you engage her in the conversation first and then let her know what is going to take place (your position in this situation, your feelings and your hopes for the relationship- whatever they are)
Ask her what would be realistic (let her know that there are other relationship that face similar situation- distance, family obligations, etc) and that they find a way to make it work out.
There is never an easy way to bring difficult info to the attention of someone you care about. Just make sure that you're patient with her and let her vent if she has to do so.