Films like The Kids Are All Right , Marriage Story , and The Lost Daughter reject the fairy tale ending. They argue that the measure of a blended family is not harmony, but resilience. It is the ability to sit at a Thanksgiving dinner where two ex-spouses are present, to comfort a crying child who still misses their "real" dad, and to wake up the next morning and try again.
In (2010), directed by Lisa Cholodenko, the family home is a biological fortress built by two mothers (Annette Bening and Julianne Moore). When the sperm donor (Mark Ruffalo) enters the picture, he doesn't just disrupt the marriage; he disrupts the territory . The kids start spending time at his fun, messy, rule-free house. Suddenly, the "new" space competes with the "old" space. The film brilliantly shows that you cannot blend a family without first blending the furniture, the bedrooms, and the rules attached to them.
If there is a thesis to modern cinema’s treatment of blended family dynamics, it is that there is no finish line. You do not "win" at step-parenting. You survive Tuesday.