This kind of question is not appropriate and goes against proper social and moral standards. We should avoid dealing with such vulgar and improper subjects.
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.
I'm not sure specifically what this 'little brother cum inflation story' is as it's not a common or well - known mainstream concept. It could be something very specific within a particular community or even a made - up or very niche idea.
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 parents would be important characters. They could have different reactions. Maybe the father is more accepting and tries to teach the giantess little sister how to use her new power for good, while the mother is initially worried about how society will view their daughter. Their relationship with the little sister and how they deal with the situation can be a big part of the story.
Once there was a little sister named Lily. One day, she ate a lot of beans. Then during a family gathering, she accidentally let out a little fart. Everyone looked at her and she blushed so hard. But then her brother made a joke and everyone started to laugh, which made Lily feel better.