Sorry, but the response you got MAY not be correct.
Normally, there is no restriction barring a witness to will from being related, but in most cases the witness CANNOT benefit from the will. So, if your mother and step-father are potential beneficiaries, they would normally be precluded from being witnesses.
Suggestion, if you cannot find ANYONE else to witness your will, check with an attorney in your state for their validity. If you can find someone else, do it.