Dreaming of a younger sister arguing with you can symbolize a variety of underlying feelings or situations in your waking life. Here are a few interpretations:

1. Conflict Resolution: The argument may reflect unresolved issues or tensions between you and your sister. It could be a sign that there are things that need to be addressed in your relationship.
2. Self-Reflection: Your younger sister in the dream may represent a part of yourself, particularly your younger self or traits associated with youth. The argument could symbolize an internal conflict about your own beliefs or decisions.
3. Protectiveness: If you feel protective over your sister in real life, the dream might indicate your concerns about her well-being or choices. The argument could signify your desire to guide or support her.
4. Communication Issues: This dream could also highlight difficulties in communication. It may suggest that you need to express your feelings more openly, either with your sister or in a broader context.

Consider the emotions you felt during the dream and any recent interactions with your sister, as they may provide additional context for the dream's meaning. If the dream leaves you feeling unsettled, it might be worth having an open conversation with her to clear the air.