One simple way is to just ignore it as long as it's not causing any real harm. Sometimes people get excited about things and it might pass on its own.
Set clear boundaries from the start. Let them know what is acceptable and what isn't when it comes to your fanfic. For example, you can say that they are not allowed to share or discuss it with others without your permission. Also, communicate your feelings clearly. Tell them that while you understand their interest, it makes you feel a bit uneasy. And if possible, try to find a common ground or a new shared interest that can overshadow their obsession with your fanfic.
Share with them some other aspects of your life that you are more comfortable with them being involved in. For example, if you like cooking, involve them in a cooking project. This way, their attention can be diverted from your fanfic.
The novel might show the main character's discomfort. For instance, it could describe how the in - laws constantly interfere in the character's daily life, like always dropping by unannounced or trying to control their decisions.
You could talk to a trusted adult in the family, like your parents. They may be able to handle this situation better. They can guide your sister and help her understand the inappropriateness of such content. Also, limit her access to sources where she might get such inappropriate stories.
You could try having an open and honest conversation with them about how it makes you uncomfortable. Explain that it's a fictional and personal interest that you'd prefer they respect your boundaries regarding.
Maybe they see something special in you. It could be your kind heart, or your unique talents. In the novel, perhaps you represent a new hope or a fresh start for the family.
It might be that they are really attached to you and think highly of you. Maybe they have a lot of free time and focus too much on you. Or it could be that they are trying to figure out how to have a better relationship with you and go a bit overboard.
Maybe they find the storylines and art styles really engaging and different from what they're used to.
It could be that the diverse themes and characters in manhwa appeal to their interests. Or perhaps they enjoy the escape and entertainment it provides from their daily routine.
One way is to practice relaxation techniques. Deep breathing or meditation can help you stay calm when dealing with their over - the - top behavior. Another is to find an outlet for your stress, like a hobby or exercise. For instance, going for a run every time they do something obsessive can clear your head.
It could be that your actions or personality in the manga have made a deep impression on them. Or perhaps there's some underlying story or plot device that explains their obsession.
Maybe it's because you have some unique qualities or traits that draw their attention and interest.