Q: Is a man a ghair mehram for his step-daughter?
A: A step-father, though being a guardian, is not a mehram for his step-daughter when it comes to Hajj, Umrah, or any form of travel. The hadith states: A woman who believes in Allah and the Last is not permitted to travel a journey of three days (i.e. 48 miles) or more without her husband or mehram. (Bukhari, Muslim)
She, therefore, cannot travel for Hajj or Umrah with the step-father only, unless there is another mehram such as her baaligh brother or nephew or uncle who accompanies them.
And Allah knows best