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.
Sharing 'cock cum stories' with your sister is extremely inappropriate. Family is a place of love, care and positive interaction. Such vulgar content has no place in family communication. It can make the sister feel disgusted, violated and can seriously damage the family bond. Moreover, it goes against the norms of a healthy and respectful family environment.
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'mom sa she needs to cum stories' is rather ambiguous. If we assume'sa' is a misspelling or abbreviation, it still doesn't make much sense as it is. 'Cum' is often used in inappropriate or vulgar contexts, but if we consider it a wrong usage for 'come', it might mean that mom said she needs to come (perhaps to a place) to tell stories. However, this is just speculation.
This is a rather strange request. It could be that she's testing your reaction or she's been exposed to some strange content recently that made her curious about this kind of stories. But again, it's important to approach the situation with respect and ask her directly what she's really after. If it makes you uncomfortable, let her know that certain topics are off - limits in your relationship.
Perhaps SRX and his little sister are at home and they decide to bake cookies together. They make a mess in the kitchen but end up with delicious cookies. SRX helps his sister with the measuring and mixing, and they share the treats with their family.