Dear in need of advice,
Talk to Your Parents
Boy, that's a difficult position to be in. My advice actually would be to talk to your parents together. BEFORE you do anything, before you become closer and more involved with each other than you are already.
You Are Going To Be Family
With your parents getting married, you have to realize that you are going to be called stepbrother and stepsister for a reason. You are going to be considered, for all intents and purposes, as family to each other. Without discrediting what you feel for each other, you really have to take a look at what it's worth...and what the consequences could be of the two of you getting together.
You are 14 and 15 years old, respectively...you have a long life ahead of you. In some form or another, that life with each other is always going to be involved as family. Your parents are getting married, and binding you to each other in that way for as long as it shall be. Your possibility of a relationship with her may or may not work out. But you will always be related as step brother and sister.
She's Worried For a Reason
A lot of this will depend on your family. If she is worried about what they will think, she probably has a point. This type of relationship isn't just about the two of you, it's about your entire family. Before you take any steps further, you really need to discuss this with your family, and make sure that your decision is the right one to make all around.
Control Your Actions
You are going to be family, and whether the marriage is a year away or not, you are still heading into a difficult position.
You can't always control your feelings, but you can control your actions. And before you act, you need to consider your entire family and what something like this could do to your future.