There are some incredibly bizarre laws and rules in Islam including breastfeeding. Apparently, if a woman breastfeeds a child that is not hers, she is then considered the mother of that child, and if she has children of her own, they are all considered siblings and are not allowed to marry. This Islamic law has reared its ludicrous head in Saudi Arabia where a couple has been forced to divorce after 25 years of marriage, and seven children later. Somehow it was discovered that both the husband and wife had suckled the breast of the same woman when they were babies.
Sheikh Abdulaziz al-Asheikh, the Saudi Grand Mufti gave the go ahead for the divorce.
So it's okay to marry a first cousin (it happens all the time in the Middle East), but not okay to marry a non-relative who has been breastfed by the same mother.
Source: Al Arabiya