Naught to do with the Child ballad "The Twa Sisters" but on making harps in folklore.

One Scottish tale (which is in a book I loaned years ago, and haven't seen again) tells of a woman whose baby is snatched away by the fairies. She journeys to the Otherworld and begs to have her child back, but is told she must trade three one-of-a-kind treasures for him. One treasure is a harp she built (I think she build the frame from the bones of seabirds -- either that or diftwood) and strung with her own golden hair.

I only half remember the story, but that image seemed to capture the essence of a Mother's love...