I don’t know what to do..

A little background:

My biological father is a drug addicted, stealing, lying, abusive man. My mother met him young and when they found out she was pregnant, they got married. 9 months later, I popped out! Now, I don’t know why my mother choose to marry him knowing what she knew about him but at that time he hadn’t been abusive. He would disappear for days on end; no phone calls, no messages..just gone. Things got increasingly worse until one day my mother decided to leave him and get their marriage annulled. Fast forward to 7 years later and she is happily married to my step-father who I love with all my heart (he raised me) with two more children (my little brothers) in a different state. She tells me about my biological father one day and that my step father was going to adopt me. I was young and blamed myself for a lot of the hurt she went through. If she hadn’t gotten pregnant, would she have married him? If she hadn’t gotten pregnant with me then she would have had to deal with him and all the pain he caused..that kinda thing. I didn’t have any desire to talk to him at all; I still don’t though I’ve had the opportunity presented to me multiple times. There are questions I have but they can wait to be answered.

Fast forward 10 years later: I found out a few weeks ago that I have a sister. I am 20 and she is 14. She is my half sister (related from my father). I haven’t personally talked to her but I had to find out about her from my cousin. My mother never told me that I have a sister. I have been thinking about all the things I have missed over the years with my sister..it bugs me. I feel very protective of her already and I don’t even know her. She has a relationship with our father but I don’t. I want to talk to her but I can’t ask her to keep me from her father (I don’t want him to know where I am or that she is having some contact with me). I know she is young and I certainly don’t want to force anything on her. I will leave it up to her to decide if she wants to get to know each other; that will be her choice entirely and I will respect her choice no matter what. I don’t know if I talk to her now or wait until she is older? What should I do?