Some things take time. The girl's mother is likely to come around if both of you have patience and persist (and pray).
However, there should be some logical thought and actions, too. I think it is all rather unlikely to work if you are in the US. You should come here first - for whatever time it can take - if this is important for you. (I am assuming you are not already here.)
There are 2 things there:
1. Firstly, I think physical proximity will make it much more difficult for the girl to reconcile with living without you, and she will try harder convincing her mother. Also, I do not think she will dump you to keep her mother happy if things come to that, after you have done such a thing for her - risked losing your job and even a career in the US for her, to spend whatever time it takes convincing her mother. It's a huge gesture, and she knows you did it for her.
2. Secondly, if you are here, you can constantly interact with her mother, do things for her (it will all be unsolicited, of course, and you will have to kill your ego a lot as she keeps in all likelihood snubbing you - I'm sure you've watched enough movies
), and slowly get her to see you as a person than as a member of a caste. All you have to do is persist - I'm sure her mother will eventually see you as a nice person who cares for her consent, and I think she will give it.
3. Thirdly, about the mother. It's possible for both of you to feel bad if a scenario comes where she has to defy her mother because she cannot leave you both out of love and out of gratitude for what you chose to do. She will feel bad for obvious reasons, and it's possible for you to feel bad since you may feel you plotted it all wean her away from her mother if the mother cannot get convinced.
It will be extremely painful for the mother to reconcile with the fact that her kid she raised all these years does not care about giving her up for someone else, and to lead a life alone after that with no one to share the grief with. So what is ethical?
I am not sure I know. I feel we all have children not to give happiness to someone who is not even born, but because we want the experience and happiness of raising children. It's for our selfishness. So I do not feel we can expect our children to give up their happiness out of gratitude for our raising them.
However, for someone who did not raise children with that perspective, and thus has expectations, the only truth is that there is going to be great hurt. And that we have the power to avoid giving that hurt to them by self-sacrifice. It's not about logical arguments about whether expectations are justified or not, it is a very real scenario where someone will be crippled emotionally for a long while if we act in a certain way.
So do we want to take the hurt upon ourselves to save the other person? It may be a supreme act of self-sacrifice given for most people lost love can affect a lot of things, but our act of self-sacrifice should not cause unfair hurt to a third party either - the girl, if she chooses to do this, will hurt you badly.
There are too many complications, and it's sad when even a big personal sacrifice isn't a way out. You have to think it all out. I would give it my best shot to persuade her mother, and pray for some divine intervention. Best of luck!