The direct cause of MS is unknown and there are many theories involving viral exposure, environmental factors, and hereditary markers.  

Just two years ago, the NMSS stated that the average person in the US has a 1 in 750 chance of developing MS.  If the same person has a parent with MS, the risk increases to about 1 in 40.  It was not stated what the risk was for a person with a grandparent with MS.

I remember reading that about 5% of people with MS have a brother or sister who is affected and about 15% have a close relative who is affected.  So it does seem that heredity plays a role, but it also means that 85% of people with MS have no close relatives who are similarly affected.

There is important research ongoing which seeks to identify genes and genetic markers which help to regulate the immune system.  Just one year ago, researchers part of the International Multiple Sclerosis Genetics Consortium announced that they had uncovered new genetic variations associated with MS.

However, even the lead researcher pointed out that "in MS, each gene contributes only a small amount of risk.  The big question is, how do they interact with each other, and are they in common pathways?"

I find this research very interesting for a similar reason for your question.  I have one first-degree relative with an autoimmune disease (not MS) and she has three siblings who have developed other autoimmune diseases.  I also have Rheumatoid Arthritis and am told that two of my great-grandmothers had RA.

Personally, I do believe there must be a genetic factor which contributes to the susceptibility of developing an autoimmune disease.  But as heredity is not the only factor, it is hard to predict with accuracy what anyone's risk is.

I do wish you the best.  I hope this helps.

Lisa