My fianc has a sister with preder willi's syndrome... what kind of risk does our future child have...is it genetic ?

It's complex: Prader Willi Syndrome is caused by the absence of a small part of chromosome 15 either through a microdeletion or through a phenomenon called uniparental disomy whereby both copies of chromosome 15 come from the mother resulting in the paternally imprinted segment being absent. A more rare situation is one in which the process of imprinting is abnormal in the father. This is because the imprinting gene is defective. The greatest recurrence risk is in this latter situation. Although the imprinting defect can be a spontaneous occurrence there can be up to a 50 percent risk that a normal male sibling will carry it and father a child with PWS. Learning the details of his sister’s chromosome studies and consultation with a genetic counselor can guide recommendations for testing your fiancee and determination of risk for your children.