‘The sisters therefore sent to him saying, “Lord, he whom you love is sick”.'
It is apparent that Lazarus was very ill, and his sisters therefore turned to the only One Whom they felt could help them. They sent Him a message, ‘Lord, he whom you love is ill'. These words emphasise the close friendship there was between Jesus and the family. Jesus is seen to be human and to have close personal friends.
The use of ‘Lord' here goes beyond just a formal greeting. It is not to be seen as over-theological in its use by Martha and Mary, but rather as an acknowledgement of the respect in which they held Him. It was probably otherwise in the mind of the writer who wants us to see Him as Lord over all.