I think you need to wait until she's out of high school for sure. It's too early to get into this with her. I agree with the others of how to tell her, and to emphasize how much she is loved, and that you and her mother are her parents. If she has an idea of this, it might be her that starts the conversation with you, but if it were me i would wait for that. If it was my son, and he had an idea, but did not bring it up to us, i would think this was because it was too early for him to deal with the subject emotionally and he didn't want things to change. So love her as you do, with everything you have to give and i'm pretty sure she'll be okay. I'm sure she knows how much she's loved and she's grateful for you both, that's all any kid could ask for.
I would second the notion that the child should be told of how much you, her parents, love her. I am not sure at 14 she should know that she was the product of a relationship that is considered incestuous in your culture.
Ah, this is hard. I think one thing to really emphasize to you here and for you to emphasize to your daughter that you ARE her dad. Bio dad or not, YOU have been her loving father. This is so important and will matter most to her. I would try to not tell her with shame attached. She doesn't want to feel shameful about her birth. Was your wife in love with her cousin? In some societies, this is not taboo as it is in others. It sounds like it is in your culture. But what came out of that union was this beautiful girl. So focus on THAT. When you decide to tell her is a judgment call. The longer it goes, I guess the better chance of anger about her being in the dark. Does the whole family know but her? Make it a loving talk and not one that you 'hate' to have. Because, this is a love story. She was born into this world and has two parents that love her dearly. A love story. Keep that in the forefront of the conversation. Let us know how it goes!!