Why do Isabel and her family feel like they are living with "some sort of god or monster?

Sonny was a surreal presence.... like he wasn't there at all.

They began, in a way, to be afflicted by this presence that was living in their home. It was as though Sonny were some sort of god, or monster. He moved in an atmosphere which wasn't like theirs at all. They fed him and he ate, he washed himself, he walked in and out of their door; he certainly wasn't nasty or unpleasant or rude. Sonny isn't any of those things; but it was as though he were all wrapped up in some cloud, some fire, some vision all his own; and there wasn't any way to reach him.

At the same time, he wasn't really a man yet, he was still a child, and they had to watch out for him in all kinds of ways.