Dreaming about your ex-girlfriend's little sister and her being in pain can symbolize various emotions and thoughts you may have regarding your past relationships or your feelings of responsibility. Here are some possible interpretations:

1. Concern and Protection: Your dream could reflect your subconscious feelings of concern for others, particularly those connected to your past. The act of rescuing her may indicate a protective instinct or a desire to help those in vulnerable situations.
2. Processing Emotions: This dream might also represent unresolved feelings related to your past relationship. It could suggest that you are processing emotions about your ex or the dynamics of your previous relationships.
3. Symbolic Representation: The little sister may symbolize innocence or youth. Her injury could represent feelings of helplessness or vulnerability in your life or the lives of those around you. You might feel a need to be supportive or to intervene in situations where others are struggling.
4. Personal Reflection: Consider how this dream relates to your current life. Are there areas where you feel responsible for others? Are there situations in your life where you feel the need to step in and help?

Reflecting on these themes can provide insights into your current emotions and relationships. If this dream continues to resonate, you might find it helpful to explore these feelings further, either through journaling or talking with someone you trust.