Abusive step-parent has Stage 4 breast cancer

Hi, I've never posted in a group forum before so I hope this is okay. I'll keep some details vague to maintain my privacy as best I can. 

My family life has been complicated as long as I can remember. My biological parents were never really in love, and when I was small my step-parent entered my life as my mother's best friend. Later on they began a relationship once my bio parents divorce proceedings were finalised. Step parent has had a very complicated relationship with my mother, myself and my sibling; she was essentially my childhood bully, playing my sibling and I off each other, making fun of us, saying cruel things, threatening to harm me... I even overheard my mother shouting at her saying "It's so obvious you don't like OP" during an argument. She's never accepted me for who I am, and her relationship with my mother has been chaotic and codependent. My mother has had substance abuse problems in the past, which resulted in them splitting up and getting together again many times. Needless to say I grew up in a home which entirely lacked unconditional love, and as soon as I could I moved to a city a couple of hours away and stared limiting my contact. I've had multiple therapists, including a trauma therapist since to try and get well. Therapy has helped me to begin repairing the relationship with my mum, who has since gotten sober and is doing a lot better, but I wasn't convinced I'd ever have a cordial relationship with step parent.

This January, step parent was diagnosed with stage 4 breast cancer after being sent to hospital with a chest infection. It was a real shock, and I had no idea how to feel. Since her diagnosis, she's started to be almost kind to me. No more horrible remarks about how I choose to look, no more stupid jokes or catty comments about how much of an idiot I am or ordering me around. I've also learned a lot about her childhood, which explained a lot of the behaviour she exhibited towards myself and my family. Now I'm in a really weird place with it all. My mum has said she's forgiven her partner for everything. On the one hand, I appreciate that partner is trying to be civil with me so we can at least try and be on good terms when she eventually passes away. It's been nice not fighting with her. Yet on the other, I'm constantly scared she'll turn on me and be horrible again someday. I'm scared for what will happen to my mother when she dies. I'm angry, hurt and resentful for everything step parent did in the past to me, for the fact I feel I'm expected to forgive and forget, and for the fact it took CANCER for her to finally look in the mirror and treat me like a fucking human being instead of a reminder that my mum had a life before her. If I mention her cancer to anyone who doesn't know about the abuse, they automatically assume I'm devastated by it and insist on calling me brave. My chosen family have all said that it's ok I'm not upset, and my best friend reminded me that I was a child and she was an adult who should have done better by me, but I still feel guilt along with all the other shit. All that means I don't really know how I feel or what to do. 

Has anyone else been through anything similar?

Thanks for reading