If someone that has the same mom but different dad have sex end end up pregnant will the baby make it?

It would still be incest, your child would also be your niece or nephew

This is too risky and not fair for the child. Why be selfish and take a chance? The world is full of people. People that you are NOT kin to. Please tell me this is a theoretical question, and not a real situation. This is NOT a stepbrother or stepsister. This is blood relation. The same DNA is in both of the people's blood.