Having an abortion when the doctor says the fetus is sick

Q: Your Eminence, I am a 31-year-old man. My elder brother died eight years ago leaving behind four children and a wife who is also our cousin. Out of my desire to raise my nephews and not to expose them to problems that might result from their young mother's marriage to another person, I sought help from Allah and married her. I did so to provide her and my nephews with an honorable life while living with my family. This was the only matter that consoled my father after my brother's death (may Allah be merciful to him).All praise be to Allah, she was a righteous wife but Allah willed that we have a genetic disease in our family that results in the death of the newborn around one or two months after birth. I buried the last child we had one month ago, and he was the third to die in the same way. (Part No. 21; Page No. 446) Due to my desire to have good children who would support me as I grow older, and as I am unable to marry again as I cannot afford a second family on my modest salary, I went to the hospital to seek necessary treatment. We suffer psychologically during the pregnancies, after delivery and upon the death of newly born children. The doctors finally referred us to the specialized hospital in Riyadh to take samples of the fetus while in the mother's womb in order to determine whether the fetus is healthy or has the same disease. If the fetus is healthy, the mother can carry on the pregnancy; and if not, she should undergo abortion. Now we seek to know whether this process is Islamically permissible, considering that this is expected to happen in the fourth month of pregnancy.What is the permissibility of undergoing an abortion in such a case? I cannot divorce my cousin or cause misery to my nephews, and I cannot marry another woman due to my financial status as mentioned. We appreciate your guidance, may Allah bless you.