After completing the Lost Child Side Quest, head on over to the Light Tower (the area where you first encounter Groose) and speak with Parrow to learn that his little sister Orielle has gone missing.
Jump off the ledge and call on your trusty Crimson Loftwing and fly Southwest towards Fun-Fun Island. On the way their you will spot a tiny island with an injured Loftwing on it; land here to find Orielle who will ask you to bring her some medicine to help get her Loftwing back on its feet. Head back to Skyloft and find Parrow who will provide you with an Empty Bottle filled with Mushroom Spores.
NOTE: If you already have a bottle filled with Mushroom Spores when you go to Orielle she will take them and cure her Loftwing immediately saving you the trip to Parrow and back. Afterwards you can still visit Parrow to receive an Empty Bottle and five Gratitude Crystals.
Fly back on over to the Sister and she will give you 5 Gratitude Crystals. What’s even better is that you can go back to Skyloft and tell the brother that his sister’s Loftwing is healed for another 5 Gratitude Crystals.Skyward Sword - Side Quest: Missing Sister,