My sister is has an intellectual disability, is this hereditary?

My sister has an intellectual disability and my brother ped away when he was a year old, he was premature and was not able to digest and swallow food. I came out normal, however, because my parents are first cousins I ume this is the reason for my sister and brothers medical diagnoses. I want to know if when I get married, will or do I have this gene in me to p onto my children?