The issue, especially these days, is NOT one of morality, as far as I am concerned. The risk is only one of spoiling a loving familial relationship by her feeling awkward whenever the two of you have to be at family functions, etc....
But that risk is small. Many many have had intimate relationships with cousins (or closer) that have either lasted for years or become life-long close arrangements. In my opinion, you MUST let her know sometime, somehow that you love her as more than just your relative. Communication is at the heart of any relationship, but HOW you are to communicate this is the question. A lot depends on circumstance. If you both are playing at "camping out" in your backyard, for example, that situation already carries a level of intimacy with it. You'll probably be undressed to one degree or another, and spend the night giggling and sharing secret desires. This, BTW gives you an insight into HER feelings about M v F, etc etc. and also opens the doors to revealing some of how you feel. You can take it from there.
But basically, you must give her some idea, some hint of your feelings for her. Take her hand. Put your arm around her shoulder or waist, in a neutral fashion; I'm not suggesting that you start right off caressing her. But she'll feel the pressure of your hand and she can read from that. Give her a goodbye kiss (on the lips) when you part, and maybe hold her gaze just a 1/2 second longer than you might. Be subtle, if you can stand it. But don't give up unless she is actively avoiding contact with you. Be sensitive to this, and back away by going back to giving "neutral" vibes when she is around. You want her to feel comfortable and relax around you.
It's not easy for any of us to read the thoughts and hidden desires of others, so keep all your senses alert to what she may be saying non-verbally to you as well.
Good Luck!