Prompt: Summer adopts an orphaned young Cinder into her family before Salem can find her, showering Cinder with love and kindness
Ruby ran up the side of the tower, trusting Weiss' semblance to help keep her moving upward. The sounds of an intense battle raged above her, but right before she reached the top, she heard a single cry and it suddenly fell silent. She put on a burst of speed and flew over the edge ready to help Pyrrha fight, but that readiness gave way to relief as she saw the woman in front of her. "Cinder!" She screamed as she launched herself at her sister. She wrapped her arms around her and hugged her as tight as she could. "I was so worried about you! We've been trying to reach you, but you never answered your scroll. What happened?"
"Ruby, get away from her." A voice said quietly behind it. It was clearly laced with an intense pain. Ruby turned and saw Pyrrha for the first time. She was on the ground, an arrow sticking out of her heel. Her worry for her friend returned, and she raced over to her side.
"Pyrrha, what happened? Who did this to you?"
"Cinder. She's working with the Grimm." Pyrrha replied, but Ruby knew she had to be mistaken. She must have lost a lot of blood and was clearly becoming delirious.
"We need to get her help now. Cinder, help me get her down the tower." She moved to pick up Pyrrha, but after a few seconds Cinder still hadn't come over to help. She looked back to see her sister standing still with a look Ruby couldn't understand on her face.
"Move away from her." Cinder said. Ruby noticed for the first time that her bow was in her hand. She more closely at Pyrrha's heel and immediately recognized the arrow. The pieces she'd wanted to deny suddenly fell into place in her mind, and she was hit with a deep pain mixed in with the horrible sensation of betrayal. She stood up and looked her eldest sister straight in the eyes.
"How could you do this?" Ruby asked, her outrage clear in her voice.
"How could I not?" Cinder asked, anger clear in hers. "How could I ever let the man who got Mom killed get away with it? How could I let Ozpin continue grinding people into dust for his war? Dad and Yang and you might be too weak willed to do anything about it, but I'm not. Nothing in this world could have stopped me from avenging our mother."
"What about all the people who are dying right now? All the other families you're ripping apart?"
"Ozpin is a cancer rooted deep in society. You can't cut him out without some collateral damage. They're dying for a worthy cause."
Ruby could barely believe her sister was saying these things. She never thought words could hurt her so deeply. "Mom would be so disappointed in you." Ruby said.
"YOU DIDN'T EVEN KNOW HER!" Cinder screamed back. It was accompanied by a blast of impossibly strong wind that slammed Ruby into the ground so hard it knocked her breath away. It took a few seconds for her to recover, and when she finally looked up, she saw a terrifying sight. Her sister, rage twisting her features and flames surrounding her eyes, holding a drawn bow aimed straight at her heart.
"Are you going to kill me?" Ruby asked. She wished she had been able to keep the fear out of her voice, but she knew Cinder would hear it.
"No." Cinder replied. "Mom did love you. And you're too weak to do anything to stop us." She turned slightly and released the string. Her arrow flew from the bow and connected with the middle of Pyrrha's chest. "She, however, could have been annoying." Ruby stared, stunned, at her murderous sister. Betrayal and outrage and anger surged inside of her, and she knew she needed to release it before it tore her apart. "Pyrrha!" She screamed. She felt her eyes begin to burn, heard her sister give out a final cry, and then the world went white.