Wow.
The decision lies ultimately in your hands. You must never forget this. While I sympathize with trying to appease your mother, I feel it's important to note that you are not responsible for your mother's happiness. This may sound selfish, but you cannot let your mother influence your decision; you must do what is right for you and you alone. Your mother's happiness lies in her own hands, not yours.
It sounds like you really love this girl, Emily. It sounds like you are looking to find a life with her. That's wonderful. I've gathered from your writing that she makes you happy.
I guess the question here is whose happiness are you willing to sacrifice? Are you willing to forsake your mother's happiness and stay with Emily? Are you willing to forsake your own happiness and give her up?
All that being said, the only person in this world that you have to please is yourself. I know that sounds selfish, but hear me out.
You are the one who will have to live with your decision, not Emily and certainly not your family. I understand the need for harmony and peace in the family, but will your happiness be the sacrifice for that harmony?
It is a pickle, no doubt about it. I think, in the end, you have to do what is right for you, not because you're trying to live up to someone's expectations to please them.
But hey, what do I know? I'm just some dope on the InterWeb thingy
I know I wasn't much help, but I wish you luck