It could be that your treehouse stories offer an escape into a different world for her. A world where she can forget about any troubles or boredom. The stories might be filled with interesting details like the treehouse having secret compartments or friendly animals living nearby. All these elements combined make your stories very attractive to her.
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.
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.
The use of 'cum' in this context is inappropriate, so we should rephrase it. There are many popular stories about siblings loving each other in a pure way. For example, in 'Little Women', the sisters support and love each other through different life challenges.
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.
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.
It's not really appropriate. Beds are very personal spaces. You should encourage her to sleep in her own bed. It can also help her develop good sleep habits and a sense of independence.