The cause of osteochondritis dissecans (OCD) remains unknown. It's not clear if it is inherited though there seem to be some family ties such as in your family. Scientists call this familial predisposition.
A few studies have shown limited evidence to support the idea of a genetic pattern. Most of the people affected were first generation (father, sibling, or brother) rather than second-degree relatives such as a nephew.
Most of the time there isn't any obvious family or inherited link. OCD does tend to occur more often with some other inherited conditions. These include dwarfism, Legg-Calvé-Perthes disease, and Stickler's syndrome.
Right now scientists suspect OCD is a multifactorial problem. Genetics may play a part in that but nothing has been clearly proven just yet.