First of all, I'm not going to make moralistic pronouncements about what the pair of you have been doing, I'll just concern my reply with the facts, and what I think you should do next.
Abortion is actually the murder of your unborn child. You love your sister, in a way that society will not accept, and the result of that love is a baby, do you feel that killing this innocent life is the right solution?
Your parents love you both, and in spite of their anger and shock they will till love you once dad has stopped raging, and mum has dried her tears, so tell them what has happened. There are ways to resolve those things without abortion, and mum and dad will help both of you.Yes, they will be furious at first but if you think you are unique in what's happened, believe me you're not!!!
As for the incidence of genetic abnormalities, it isn't a foregone conclusion. I know personally, a brother and sister who live as husband and wife, as far as everyone else is aware. They have two beautiful healthy and perfectly normal daughters, and have a very loving and happy life together. All their friends and neighbours assume they are a married couple, and I have no right to destroy their lives by breaking their trust, so I keep quiet about it. I relate this info only because nothing I've said would reveal their identity, nor will I ever do so.
It's a fact of life that humans all n love with each other, and sometimes it happens to two people that live tuner the same roof, and sometimes to brothers and sisters, love doesn't always follow the rules!!! You and your sister have been either stupid, or unlucky, in her getting pregnant, but the consequences demand that you tell our parents the truth, please don't act like cowards, or try to get your sister to pretend the child is someone else's, so hat you escape the wrath of the family. You thought you were a man when you started this, now it's time to step up and be a real man. I can imagine your sister is frightened, both because she is pregnant, and because it is the result of your incestuous relationship. You need, even more than ever, to give her your love and care, because she will need every ounce of your love to help her to cope with the rage of all around her. In fact, you'll need each other more than ever before!!!
I expect your family will try to force he wo of you apart, but you and your sister need to decide what you both want for the future.
I wish you both good luck.
Love,
Salma