We use big sis/lil sis in my Theta chapter, and we get them a few weeks after initiation...I guess we get them pretty late, but on bid day we get a guardian angel to help us out during the pledge period and until we get our big sis.
Funny story, the other day my mom told me to tell my "big sister" how much we enjoyed the cookies she gave me to take home, and for a second I was really confused, because I think of my "big sister" as my biological older sister, and my "big sis" as my sorority sister. I was like, um, why don't you tell her yourself, she's your daughter! And then I realized how much of a difference that one little syllable makes in my head
Sarah