If a mother breastfeeds a baby who is not of hers, this baby becomes a sibling with mothers own babies. Some says, to be a "sütkardeş" the age of the breastfed baby must be under 2. Since they are siblings then, the rules for marriage between legal siblings apply for "sütkardeş"s too. This is not a law, but general attitude towards the relations between "sütkardeş"s is so. There is also an old, famous comedy movie called "Sütkardeşler". Worth watching :)

It just means milk siblings, nothing else.