This is one of those plotlines that definitely falls into the 'rare but not nonexistent' category in films. While it's not a mainstream trope, there are a few titles that explore similar territory, often within the realm of dark comedies, family dramas, or even thrillers. The dynamic of a step-sibling relationship adding complexity to an unexpected pregnancy can make for some intense storytelling, blending emotional conflict with societal taboos.
One example that comes to mind is 'The Pregnancy Pact,' though it's more about teenage pregnancy than step-siblings. For something closer to your query, indie films and certain foreign productions occasionally dabble in these themes, often pushing boundaries to explore familial tension. It's the kind of premise that filmmakers might use to dissect themes like guilt, secrecy, or the blurred lines between modern family structures. If you're digging for recommendations, I'd suggest checking out forums dedicated to unconventional movie plots—they sometimes unearth hidden gems with these very specific setups.
Personally, I find these narratives fascinating because they force characters to confront messy, real-life emotions in exaggerated scenarios. It’s not everyone’s cup of tea, but when done well, it can be oddly compelling.