I found this post on the African Loft, and since it won't let me comment unless I'm a member, then I'll do my commenting here.
Okay, let's look at this objectively. There's no need berating you about condoms because it won't change what happened. (Note to Misi: A classic example, of sexual blackmail in relation to the Obasanjo post). Yes back to the distressed teen. The situation is not as bad as you think. You have a number of options. If you are afraid of ruining your sister's marriage, then you could tell her that some guy raped you, and you were to afraid to tell her before you found out you were pregnant. Then you can give birth to the child in peace and give it up for adoption. You should also tell your sister's husband/your Uncle (whichever he is) that if he ever comes near you again, you will tell your sister EVERYTHING, no matter what might happen afterwards. This should buy you some time. You will eventually have to tell your sister, but if it will be better for you not to tell her the entire truth now, then a half truth that will benefit both of you might be better