Sorry i was distracted for a moment. This really heavily depends on your relationship with sis. If mom will not stop her from seeing the older girl, you might sit down with her and explain how much you love her and carefully without condemnation explain how you have observed the changes in her because of her friendship with "D". Then point out girls you know your age like "D" and explain this is what "D" is headed toward. Then ask Is this where you want to be. Also I must mention your lifestyle will effect your credibility with her. If you hang with wild girls etc. Your word will carry little weight.