Possible Clues:

• Li'l sis, for example
• Neighbor on a family tree, informally
• Bunk bed sharer, often
• Little sister or brother?
• First roomie, for some
• Kim to Kourtney, or Kourtney to Khloé
• Hand-me-down source, for short
• Blood relative
• Quad, to another quad
• One of a quint's four