First, there are the preliminaries that will get her attention. Dinners, flowers, letters (real letters not e-mail) and so on are always a good start.
But that is like putting primer on the wall. It really is necessary if you want a long-lasting paint job that holds us. Both members of the couple have to be prepared for what you are about to say. So the preliminary events i mentioned are just getting the two of you ready for the day you simply say, "I love you, or I hope you know how much I love you." The words are not written in stone, but they have to be honest, sincere and spoken with truth and conviction.
I met my wife 33 years ago. I could have proposed that night. I realized that might be rushing things. We dated for about three months before I told her and four months later we were married. Just celebrated 32 years last month.
As in many cases, some family members do not always care for the person, you choose. My mother was ill (ALS) and my sister really did not want to see me get married at that time--afraid I would not help out with my mother.
Years later, when I was sick and my sister saw how much my wife did for me and for others, she wrote to me and said she was glad "I had the good sense to ignore her and others and marry my wife. She is a real jewel." I could not agree more.
Good luck to you.