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.
A good approach would be to educate her about the body in a general sense. Start with basic body parts and functions. Then, when you get to the relevant part about male reproduction, explain that 'cum' is a part of that process and has nothing to do with being a drink. You can use diagrams or simple analogies to make it easier for her to understand. Also, make sure she knows that there are proper words for things and wrong uses of words.
First, you need to have a proper conversation with her. Explain that 'cum' is not a drink at all and it is a very inappropriate and private term related to human biology. Maybe she heard it wrongly or got misled. Use simple and clear language to correct her misunderstanding.
This is a very inappropriate situation. First, you should immediately stop reading such stories in her presence. Then, have a serious conversation with her about appropriate and inappropriate content. Make sure she understands that this kind of content is not suitable for her age.
It's not a common phrase. Maybe it could mean 'told all of my little sister's stories' in a rather strange way. But without more context, it's really hard to be sure.
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.
Loving your sister in a proper, family - affectionate way is normal. But the term 'cum' is inappropriate and vulgar in this context. We can talk about heartwarming stories of siblings loving each other. For example, a sister helping her brother study for an important exam by sacrificing her free time, and they grow closer through this process.