Despite rumors to the contrary, this song is not about Idol's actual little sister. "Little sister" is slang for girlfriend. He is singing about a woman/girl he loves marrying someone else while he still loves her.
Idol did have a sister who was getting married, but on an episode of VH1 Storytellers
, he explained that his sister's wedding simply gave him the idea for the song. Like many of Idol's compositions, he started with the title and wrote the song from there.
Jon - Atlanta, GA