If you know you're a really good guy and treat her daughter with the greatest of respect and are simply in love and want to get married, then it will take time to win over the mother. There could be many reasons why she doesn't like you: (nothing personal against you, just an educated guess.)
I could go on, but you get the idea. If you can't provide properly for this young woman; if you have no future goals; if you treat your girlfriend with disrespect or any member of her family then do this young woman a favor and don't marry her. If you're one of the good guys, then take your fiance and sit down with her in front of the mother and both of you tell her that she will be furthering her education and you are getting married and she is welcome to be part of the wedding plans and if she doesn't want to that's fine, but if she causes anymore problems then she won't be welcome to the wedding. This will shock her into realizing the two of you mean business. Be sure to express to her that you will look after her daughter to the best of your ability and squelch her fears of not seeing her daughter that often or her daughter will not continue on with her education.
I've been married twice and I always kept in contact with my mother (she has since passed away.) My mother didn't lose a daughter, she gained a great son-in-law the second time around. So, be patient and once married help her mother out where you can such as far as fixing things around her home, etc. Hopefully in time she will realize you aren't the bad guy.