Grandma Hutto doesn't want Oliver going after the Forresters and probably getting himself killed. The Forresters don't always fight fairly, and Grandma knows Oliver wouldn't stop until he had tried to kill every one of them. There are more Forresters, and one of them would most likely kill Oliver before he could kill them. She wants to move to Boston the next day because she knows if she stays, Oliver will find out that the Forresters did start the fire, and she would rather have him alive than live where she has all of her life. Plus, she will get to see more of Oliver since Boston is usually where he ships out of.