Talking from the perspective of having being a HIV/AIDS counsellor in Africa,this is my advice to you.
It does seem as if you're mom is in denial but this is the time when she needs you the most. Talk to her and make her understand that you want her in your life and her grandkids lives (if she is a grandma)for a long time. Also let her know that no matter what the results are, you will always be there for her and it is a family matter that does not need to be discussed with anybody else.
I am surprised that the hospital or clinic that diagnosed your step-father did not try to contact the women he has been with to encourage them to get tested. Unless he refused to say and in which case it is your duty to call that clinic and talk to a counsellor about your mom. She needs to be talked to by a professional that will assure her that AIDS is not the end of the world as long as you live right. If she has it (I pray God she doesn't) all that will change is her lifestyle. Her diet will consist more of food that builds the immune system because HIV destroys the immunity we have in our bodies. She will probably be on a whole bunch of medication (called cocktails). But get this, IT IS NOT THE END OF THE WORLD.
You and her are in my prayers.
