No. 'Cum' is an inappropriate or at least very unclear term in this context. A proper English sentence should use more common and appropriate words. This phrase seems rather odd and not in line with standard English usage.
The phrase is really unclear. 'Sister watches' is simple enough, but 'cum story' is not a common expression. It might be some sort of very niche or even mis - worded idea. Maybe it was intended to be something else and got written wrong.
The title 'cum for big sister story' is rather ambiguous. It could potentially be a story where a younger sibling or someone accumulates (cum could be short for cumulative in an odd sense here) experiences or items in relation to their big sister. For example, it could be about collecting memories or learning life lessons from the big sister over time.
Since I don't really know what this story is exactly, it's hard to say where you can find it. It might be a self - written or very niche story that could be in someone's personal collection or on a very specific, perhaps local or family - based, storytelling platform.
I'm not sure specifically what this 'sister leaks cum wedding day story' is as it's not a common or well - known general topic. It could be some very specific, perhaps made - up or niche story within a particular group or community.