Juliet is Butler's sister. She is sixteen and she is training to be a bodyguard just like Butler.
Artemis does get the gold at the end of the first book. We can know this for sure because of what is said at the beginning of the second book. There, a reference is made to the ransom gold that Artemis stole from the fairies.
We also know that Artemis got away with the gold because Holly comes back out of the house and says that she has only gotten half of the gold back.
His dad (we later find out) has gone to Russia to try to deal with the Mafia up there so he can get enough money to, hopefully, cure his wife.