Dan seriously...you shouldnt refer to her as your "little" sister, although she is younger..."little" has the connotation of 14 or younger...Alyssa is 20? i think....anyways, HI ALYSSA!!! (what's your username?!)
little: adjective- 1) lowest in importance or rank.
2) slightest or tiniest. I do believe little sister does fit, because she is a lower rank in the family, being the youngest.