1. Sometimes Stevie had fewer songs than Lindsey and Christine, but her songs were usually longer. On Tusk she only has 5 songs, but they occupy more space then Lindsey's nine songs.
2. Smoking and drugs can change your voice, but not that much. Stevie's voice changed because she wasn't singing properly until the '90s. I don't know the right term, but if you don't sing properly, lumps grow on your vocal chords. You can have them removed, but after a while they grow back. Other indicator was her thick vibrato. When you don't use your diaphragm you can't create a natural vibrato.
3. She was too busy with her solo career, and so were Lindsey and Christine. They even had trouble to get her in the studio for the Tango album.
P.S. Lindsey's voice changed too.
