Research studies focusing on genetic factors in schizophrenia have found that

I believe the answer is:  an individual with a schizophrenic identical twin has the highest risk factor (almost 50%) of developing schizophrenia.

Even though environment played a factor in creating schizophrenia, Genetic variations played a larger role in causing of the majority of schizophrenia cases. In identical twins, the genetic composition between the two siblings would be extremely similar.
So when one of the siblings develop a schizophrenia, the other siblinhavehas a really high chance of also developing one.