The Sisters Projects

What had the two sisters done together

1: Haylie wrote or co-wrote several songs on her sister Hilary's album Metamorphosis
    (2003) and Hilary Duff (2004), co-producing two on the latter.

2: In Year 2004 - "Our Lips Are Sealed" — A Cinderella Story soundtrack

3: In Year 2006 - "Material Girl"— Material Girls soundtrack & Girl Next CD

Other Songs together

4: Same Old Christmas (feat. Hilary Duff)
5: The Siamese Cat Song (with Hilary Duff)
6: The Siamese Cat Song [Cat-Scratch Remix] (with Hilary Duff)