Hello there and welcome to our Moyamoya family!  You will find that all of us here are very helpful and will try to answer any questions that you might have.  Please make sure you read as much as you can on this website as it is very informative and can help you a great deal! 

You asked about the hereditary factor of Moyamoya.  I will tell you what I tell everyone else.  It is about as common as having moyamoya (not very much - 1 in 2 million people in the U.S.)  If you need a number....about 7%.  Currently, my brother is being tested for moyamoya as he has been having some symptoms like numbness in the fingers.  But they are also checking him for early stages of carpal tunnel syndrome.  And yes, it is possible that your nephew's father has been misdiagnosed and it would probably be wise to get him to have a MRI/MRA to check it out.  I would also ask the father if he has had symptoms of MM like numbness in the extremities, weakness on one of both sides of the body, headaches, seizures, migraines, slurred speach, etc.  When I first went into the hopsital with MM symptoms, they tested me for MS as well.  Fortunately, they didn't diagnose me with it.  And yes, you can develop MM later in life.  I'm 34 and was diagnosed when I was 33.  There are many people on here that are older and have MM. 

My advise is to get him tested now!  It certainly can't hurt! 

Michelle