Okay speaking as someone who was the 17 year old with divorced parents let me give my 2 cents.

Your right you arent stepmother your dads wife, and thats 100% okay and a good thing on its own, my divorced mom started dating a guy when I was 17 I wasn't looking for a dad I had my dad my entire life, I may not live with him anymore but he is my dad, and I didnt need a step dad, I didnt need someone else trying to parent me and he never tried, he's great and I like him in his own way but he's not my dad or step dad he's just moms boyfriend/husband.

He's a great guy, he's very outdoorsy and helpful, he's a wonderful addition to the family and I genuinely love him for my mom. He will technically be my step dad but...hes not my step dad, i was in college full time and I had a job full time by the time I met him, I may have been 17/18 and still living with mom but I was doing my own thing, I had responsibilities and relationships to maintain and he never tried to intrude on that.

NTA but for the sake of your husband and his daughter your only gonna make it harder by fighting Kim, just block her dont communicate if they want to play pretend let them u don't need to involve yourself. Advise Ava never to maintain a relationship she must puppet for like the one she has with her mother and be clear to say she has the right to set boundaries especially now as she's becoming an adult not parent advice just good advice for everyone.