Dreaming about your ex-girlfriend's little sister and her experiencing pain can symbolize feelings of concern or unresolved emotions related to your past relationship. Here are a few interpretations of your dream:

1. Concern for Others: Your dream may reflect your innate desire to help those in need. The act of rescuing her could indicate that you have a nurturing side and feel empathy towards others, especially those who may be vulnerable.
2. Unresolved Feelings: Since she is related to your ex, the dream might suggest that you have lingering feelings about your past relationship or unresolved issues that need addressing. It could be your mind's way of processing emotions tied to that chapter of your life.
3. Symbol of Innocence: Your ex-girlfriend's little sister might represent innocence or childhood aspects of your life. Her injury could symbolize your fear of losing that innocence or a worry about the well-being of those you care about.
4. Personal Growth: Helping someone in distress in a dream can also symbolize your own journey towards healing and recovery. It may reflect your desire to mend past wounds, either in your relationships or within yourself.

Consider how you felt during the dream and upon waking. Those emotions can provide additional clues about what this dream might mean for you. If this dream continues to resonate, you might want to reflect on your past relationships and any feelings you may still have regarding them.