Understanding this removes the "ick" factor. She wasn't doing this to you. She wasn't thinking about you at all. She was thinking about her own private fantasy world, and you accidentally intruded. Reframe the narrative: You are the intruder (even if accidentally), not the victim.
"Hey, Em, are you in there?" I asked.
: If what you've discovered makes you uncomfortable or affects your living situation, it's okay to set boundaries. Let your stepsister know how you feel and what you're comfortable with in your shared living space.
Here are three concise thumbnail/title feature options (tone variants) you can use—each includes a short title, 1-line hook, and suggested thumbnail text/visuals.
In the hours following the incident, your brain will try to "solve" the problem. Most of these solutions will make everything worse. Avoid the following at all costs: