I am sorry that you are in the situation that you are and that your childhood was ruined by what appears to be your mother's codependency and choice of a "bad" partner.
BlueRose made a sound suggestion that you relocate yourself to a place away from this "family." The first reason to do this is to remove yourself from the influence of the what created the traumas - the abusive step-father. The next reason to do this is to "Find Yourself," on your own. Who you are and whom you are intended to be is determined by you, alone, and not a series of abuses.
How to forgive? Well, my personal belief is that "forgiveness" is misinterpreted as meaning to "forgive and forget." I don't believe that this is what forgiveness is mean to be. And, it takes a fair amount of time and processing the wrongs that people have committed against us to get to that point - it is not an event that occurs just because we want it to occur, nor because others insist that it's required. This is a spontaneous event that happens when we realize that we had no control over what others chose to do to us. Forgiveness is for ourselves, alone, and NOT for the people who have intentionally harmed or betrayed us.
There is a grieving process that is well documented in a book titled "On Death And Dying," by Elizabeth Kubler-Ross. I had the fortune to study this work during high school, and I'm grateful that I did. It details the steps of grieving, and it's important to understand that those steps don't necessarily go in precise order - they often go back and forth until "Acceptance" is achieved.
"Acceptance" is the forerunner of "forgiveness," in my most humble opinion. Acceptance is that concept that we cannot change, alter, renegotiate, bargain, or force facts and truths to be anything other than what they are. Your mother made bad choices - you cannot change that fact, nor can you bargain it into something more comfortable or pleasant. The step-father is an abusive, manipulating thing - that fact cannot be altered. Your childhood was unfair, unpleasant, and full of trauma. This is another fact that cannot be changed. So, what to do?
Recognize that you have control over only one thing in this entire lifetime - yourself, your choices, your actions, and your decisions. At this moment, you have an entire lifetime ahead of you and you are very precious in this vast Universe. Make your choices and decisions based upon facts and common sense, and express your emotions through writing, singing, your culinary arts, or any other means that will purge them in a healthy, productive manner.
I type this, often, but getting involved in some strong counseling therapy with a specialist that focuses on childhood traumas will be of tremendous benefit, in the long run. I type this from personal experience - the help and guidance of a trained professional has been priceless to me in processing my life's traumas and emotions. Please, consider this as an option when you relocate yourself to a SAFE place.
Here are a couple of websites for you to visit that might help you on you personal healing path: http://www.180rule.com
There is a tremendous amount of information on healing, processing, and "acceptance" on both of these sites.
One other thing to keep in mind - shared DNA does NOT make people "family." "Family" are those people who surround us with love, support, guidance, and encouragement, and they do not have to carry the same DNA that we do.
My most sincere blessings to you on your healing journey.