NTA. Your responsibilities are to yourself first and then your bf, and finally onto whatever family your bf is close to. Considering the person involved to your bf, is only a Tertiary member of the family and has been removed from the actual family itself (or more accurately never a member of the family at all). Perhaps have a down to earth discussion with your bf to understand if the ex-step gf is worth the trouble. Tell him upfront the you have an issue with her solely due to the fact that the ex- gf/stepmom treated him as lower class compared to her children. Just explain to him that you’re edgy due to her treating him less than fair and that you consider yourself extremely protective of the people you care for the most.

As a person the has been with my spouse for 30 years, honestly is a key virtue to having a long term successful relationship. Don’t be afraid of opening your heart to him.

I’m wishing the the best of luck to help resolve this issue.