Could you mean a song with the phrase:

“Brother, you don’t know, sister, you don’t know”?

I’m not sure of the exact title from that line alone. If you want, I can help you identify it if you give me any of these:
- a few more lyrics
- the singer’s gender/voice
- the genre
- where you heard it
- whether it’s an old/new song

If you want, I can also suggest possible songs that match that lyric.