Pearljam. This song is song by 8-9 years old kid and not 20 years old brothers and sister.  So there is nothing wrong with the song when one brother shows love to sister. And majrooh sultanpuri may not be a boy but while writing a song he did think like a 9 year old brother.

And I think  you should not take all the words literally. " Sang Rahena hai....." does not mean living together in that film. May be together emotionally. If you see that movie you will probably know what that word really means.