OP, here's the thread I referenced. Situation is different (stepdad won't help with daughter's education costs, while his high income is included in the FA calculation), but you can see what people said about the decision to get married negatively impacting the FA. Maybe you can use that as a reason to put the kibosh on the wedding for now and frame it as helping both kids out.
https://www.dcurbanmom.com/jforum/posts/list/982311.page