Yes, it´s true.
Muhammad married his son´s wife Zeinab who was also his first cousin. This sunna is so popular in muslim countries that over 50% marriages are with first cousins.
The results are devastating.
Nicolai Sennels is a Danish psychologist who has done extensive research into a little-known problem in the Muslim world: the disastrous consequences of Muslim inbreeding brought about by the marriage of first-cousins.
The practice of marrying close relatives, which has been prohibited in the Judeo-Christian tradition since the days of Moses, was sanctioned by Muhammad and has been going on now for 50 generations (1,400 years) in the Muslim world.
According to Sennels, close to half of all Muslims in the world are inbred. In Pakistan, the numbers approach 70%. Even in England, more than half of Pakistani immigrants are married to their first cousins, and in Denmark the number of inbred Pakistani immigrants is around 40%.
The numbers are equally devastating in other important Muslim countries: 67% in Saudi Arabia, 64% in Jordan and Kuwait, 63% in Sudan, 60% in Iraq, and 54% in the United Arab Emirates and Qatar.
According to the BBC, this Pakistani, Muslim-inspired inbreeding is thought to explain the probability that a British Pakistani family is more than 13 times as likely to have children with recessive genetic disorders. While Pakistanis are responsible for three percent of the births in the UK, they account for 33% of children with genetic birth defects.
The risk of what are called autosomal recessive disorders such as cystic fibrosis and spinal muscular atrophy is 18 times higher and the risk of death due to malformations is 10 times higher.
A BBC report discussed Pakistanis in Britain, 55% of whom marry a first cousin. Given the high rate of such marriages, many children come from repeated generations of first-cousin marriages. The report states that these children are 13 times more likely than the general population to produce children with genetic disorders, and one in ten children of first-cousin marriages in Birmingham either dies in infancy or develops a serious disability.