Sleepwalking IS hereditary.
More Info: Previous studies have noted that sleepwalking runs in families, but a new study has found that it is caused by a mutated gene passed down from parent to child.
Role Genetics Play in Sleepwalking
Doctors have long noted that sleepwalking tends to run in families and that some people seem to be more susceptible than others are. Several studies set out to base this theory on scientific fact and revealed that there does seem to be a genetic basis involved with sleepwalking. A study published in the British Journal of Psychology concluded that a first-degree relative of a sleepwalker is ten times more likely to sleepwalk than the rest of the population.  A separate study published in the journal Neurology concluded that twins are more likely to sleepwalk. 
Role Genetics Play in Sleepwalking
Noting that there does seem to be a genetic link in sleepwalking, researchers from Washington University School of Medicine explored the physiological reasons why this occurs and found the answer in a faulty chromosome.
The researchers set out on a genome wide search to explore the theory that genetics plays a role in the sleep disorder by monitoring the DNA of a large family of sleepwalkers. What they discovered was that all of the sleepwalkers in the family had particular genetic code that the non-sleepwalkers did not. Moreover, that those with the faulty DNA have a 50% chance of passing it to the next generation.
Though the researchers did not find the exact location of the mutated gene, they do know that it lies somewhere in chromosome 20. 
Each cell in the body is comprised of 46 chromosomes that are divided into 23 pairs. Each parent passes along a copy of chromosome 20 to form one of the pairs. Chromosome 20 makes up 2% of the DNA in the body’s cells and contains 700-800 genes that perform a variety of roles in the body.  The researchers are looking at 28 possible genes but suspect that the mutated gene that is inherited and causes sleepwalking may be adenosine deaminase, which has been linked to slow-wave sleep stage during which sleepwalking occurs. They also theorize that because most people outgrow sleepwalking, several other genes must be involved. 
Why Is It Important to Locate the Cause of Sleepwalking?
It may seem like overkill to attempt to locate the exact gene that causes sleepwalking when for most people it is a benign condition and will be outgrown by puberty. That is not the case for everyone. Some sleepwalkers participate in dangerous behaviors, such as the 12-year girl that participated in the Washington University School of Medicine study. When she sleepwalks, she leaves the house and roams the neighborhood completely unaware that she is doing it, with no memory of the incident when she wakes up. Several high profile murder cases have used the defense that the defendant killed while sleepwalking.  For some, these parasomnia behaviors dramatically interfere with their lives and can even be life threatening.
Once science understands exactly which gene causes sleepwalking, treatment can focus on the cause and not solely the symptoms.
British Journal of Psychiatry, Kales A
Hereditary factors in sleepwalking and night terrors
1980 Volume: 137, pages: 111–118
 Neurology, Hublin C
Prevalence and genetics of sleepwalking: a population-based twin study
1997, Volume: 48, No: 1, pages: 177-181
  Neurology, AK Licis
Novel genetic findings in an extended family pedigree with sleepwalking
2011, Volume: 76, No: 1, pages: 49-52
 Genetics Home Reference
 Psychology Today
Sleepwalking Killers-And What They Tell Us about Sleep
Glossary of Terms
Chromosomes: Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.
National Human Genome Research Institute
DNA: DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms.
Genetics Home Reference
Genetics: the study of how, in all living things, the characteristics and qualities of parents are given to their children by their genes.
Genome: one haploid set of chromosomes with the genes they contain.
Hereditary: determined by genetic factors and therefore able to be passed on from parents to their offspring or descendants.
Parasomnia: refers to all the abnormal things that can happen to people while they sleep, apart from sleep apnea.
National Sleep Foundation
“Our data appear to fit a ‘two threshold’ multifactorial mode of inheritance. This finding supports the hypothesis that sleepwalking and night terrors share a common genetic predisposition, with sleepwalking being a more prevalent and less severe manifestation of the same substrate that underlies night terrors. Heritable factors predispose an individual to develop sleepwalking and/or night terrors, but expression of the trait may be influenced by environmental factors.”
A Kales British Journal of Psychology