How do I get my sister's songs from her iTunes library into my iTunes Library?

My sister has a bunch of songs that I want in her iTunes Library, but I don't know how to get them onto my laptop. I tried e-mailing them to myself, but it didn't work. Then I tried to authorize my computer, or whatever, but I don't know how to do it. Can someone explain to me how to either do the authorization thing, or another way that would work?