How did the Morton's already know about the murder?

They were not related in any way to Miriam. The police would not have known about Guy's connection to the Mortons but that should not matter.

Did not live in the same town that the murder occurred.

Things take time. A woman is found dead. The coroner pronounces her dead at the scene or in the hospital. The police then start questioning everybody at the carnival and they start looking for relatives to contact. Guy is most likely the first to be notified but he does not live with her.

So again how did the Morton's know that soon about Miriam?

Also very damning to me that Guy was not located soon. He was not home to receive phone calls and not home when the police came to notify him.