Is autism hereditary?

My husbands younger brother is severely autistic. There is no other history in my husbands family of someone being so severely handicapped. So my question is, if we have a child, are our chances of having an autistic child increased? We already have one healthy child but I have a fear that if we have another he or she will be autistic. Please only serious answers and from those who can offer fact based answers. Thank you so much!