Are step children listed on a family tree?

If the family tree is used to trace ancestry, I would think not.

I do agree with George_1950 but I would also pose the question, "Why you are creating the family tree?". If you are doing it for ancestry, heritage etc. then you would not include step because then the family's heritage would be inaccurate. If you are doing it just to show during the life of who was married to who and who was related through marriage then, yes, you would add step.