In the comments you mentioned that it was song lyrics from the song 'Sister moon' by Sting. The lyrics seem to be talking about how he spends lots of nights looking at the moon, and enjoying the moon's company and longing for the moon. Using my better judgement however I would make the assumption that it's not the moon he is talking about, rather someone in his life. He is therefore making a metaphor out of the moon. The moon may be symbolic of a woman he longs to be with? Who knows? Essentially the meaning is down to interpretation as is the way with the best song lyrics... If you want a full proof answer though, you'll have to track down Sting and ask him personally what he meant when he wrote the lyrics!

You really need to know the context in which it was used. It could be used in astronomy to describe one of the moons of a planet by reference to another one. For example, 'Ganymede is a sister moon of Europa in the Jovian system.'

"sister moon" is a song title of musician Sting. And for example, "Johnny, take a walk with your sister the moon."

Do you have the whole sentence?