A song for a sister is the song the rest of the family didn't know how to write. She was YOUR sister, not theirs. The fights, the late-night calls, the time she saved you, the time you saved her. She lived in the specific shape of a sibling relationship — and that shape is what the song should hold.
What this kind of song holds
These songs hold the inside language — the nicknames, the running jokes, the fights that the parents never knew about. They hold what siblings know about each other that no spouse, no child, no friend ever quite gets. The forge writes them with that specific frequency.
What to put in the intake
Give us the year you were closest. Give us the year you weren't. Give us the way she said your name when she was teasing you. Give us the thing nobody else would know about her. The song lands hardest when it sounds like ONE PERSON wrote it for ONE OTHER PERSON.