This disorder is autosomal recessive, which means that each affected patient inherited one mutation from each parent. The parents who have one mutation are healthy and are carriers of the disease (can pass it on without having any symptoms). That means that for each couple that have an affected child together they have a 25% chance that each additional child will be affected.
If the parents are related, and there are other family members who are also related, they are at risk to also have affected children.
If the patient’s brother or sister is a carrier (67% chance), then they have a low risk of having an affected child, unless they have children with someone they are related to.