You know, I see that your girlfriend is not going to leave her mom, isn't she? Well, the problem is that you can't change a person (her mother), try to talk with your girlfriend about your relations (I mean the three of you). What is more important for her: living all her life with her mother or changing her own life for the better?
There is too little information you gave, but on the other hand, you have to decide for yourself what is major for you. If you love her so much and don't imagine your future without her, then try to take everything as it is, though it's a hard thing to do... The only possible decision is to take your girlfriend from her mom and move to the other place. Otherwise your life will be a nightmare! If you do nothing about it all, well, you have only yourself to blame afterwards for all possible complications... Anyway, it's for you to decide.