I think my stepmom is cheating on my dad.?

You shouldn't tell your Dad anything because you don't know for a FACT that they're up to something. All you will do is make your Dad stressed while he's away working and you will PO your stepmom who you have to live with when your Dad is away. She could make life very miserable for you. But the most important reason not to say anything is because you don't KNOW for sure that anything is going on. It is not your business - it is between your parents.