Artemis' mother is unable to deal with Artemis' father's absence. She and Artemis have been led to believe that her husband and Artemis' father is dead. I don't want to give it away, but Artemis' dad does show up again later on...then his mother makes a miraculous recovery. Right now, she is suffering from massive depression and unable to deal with reality without her husband. Therefore, Artemis has free-rein of the household and is able to cook up all his wacky schemes.
Its more of a mental sickness than anything else, just like what amy-lepore said