How to share families itunes library to iphone 4 without syncing?

I have my iphone 4 synced to my Macs itunes library and I want to "share" my moms itunes library music to my iphone WITHOUT "syncing" my phone to her itunes account. Is this even possible? and how would I do this?

I remember "sharing" my cousins itunes music onto my ipod without syncing and removing all my existing music. Is it different with iphones and ipods??