Dreaming of a scenario where your mother is beating your deceased sister can be quite distressing and may represent complex emotions you are dealing with regarding your family dynamics, loss, and perhaps unresolved feelings about your sister's passing. Here are a few interpretations to consider:

1. Conflict and Resolution: This dream may signify an internal conflict or unresolved issues within your family. The act of defending yourself could indicate a struggle to assert your feelings or beliefs in the face of family pressures or unresolved grief.
2. Guilt and Responsibility: You might be grappling with feelings of guilt or responsibility related to your sister's death. The dream could reflect your subconscious processing these feelings and your desire to protect loved ones, even if they are no longer present.
3. Emotional Turmoil: The dream may symbolize emotional turmoil or distress you are experiencing in your waking life. It could be an expression of frustration or anger towards family situations or unresolved grief.
4. Fear of Loss: This dream could also stem from a fear of losing more family members or a fear of confronting painful memories. Your defense mechanism in the dream might signify your desire to hold on to your loved ones and protect your emotional well-being.

To gain further clarity, consider reflecting on your current waking life circumstances and how they might relate to the themes of conflict, loss, and protection. Journaling about your feelings or discussing them with a trusted friend or therapist might also be helpful in processing these emotions.