Could you mean a song with lyrics like “brother you don’t know, sister you don’t know”?

I’m not sure of the exact song from that phrase alone, but I can help you find it if you give me any of these:
- a few more lyrics
- the genre
- male/female singer
- where you heard it
- whether it’s old/new

If you want, I can also search likely matches from that lyric fragment.

The Earth is about 4.54 billion years old.

If you want, I can also explain how scientists figured that out.