What can I do about my step brother and his girlfriend impersonating me on facebook and harassing me? Can I take legal action ?

Sounds like a good claim for identity theft as well as some civil causes of action, such as misappropriation of name/likeness, which would allow for recovery of money damages. This answer is given in accordance with the laws of Virginia and may not be applicable in any other state.  It should not be construed as legal advice, as that would require a more thorough analysis of all of the facts involved in a specific case.  If you need further information or assistance, please feel free to contact my office for a consultation.