My brother married his first cousin (our mother’s sister’s daughter) two years ago. They had a son 11 months ago who, after an MRI scan, has been diagnosed as having Leigh’s disease, a deterioration of the brain stem. The boy suffers from the destruction of his inner organs, loss of muscle movement as well as loss of sight and hearing. Research findings indicate that death normally follows shortly after this disease develops in the brain. All indications are that, depending on the extent of mutated cells found in this boy’s muscle tissue, at least one in every four of their children, will be born with this defect.
With such odds, what is your ruling regarding my brother having further children?

Marriage to close relatives is not recommended in Islam. However, tell them that it is not forbidden – haram – for them to try again.