A standout example is (2019). While not about a traditional step-family dynamic, it explores the layers of obligation to blood versus the family we choose. This theme is also present in indie darlings like "The Kids Are All Right" (2010), where the children of a lesbian couple seek out their sperm donor father. The film explores how introducing a "new" parental figure into a stable family unit disrupts the equilibrium, forcing the "original" parents to re-assert their relevance and love.
There’s a term I didn’t know I needed until recently: conjugal stepmother . It sounds formal, almost legal, but for me, it simply means “home.” my conjugal stepmother julia ann