I need help remembering a children's book?

My teacher read it to us in Elementary School. It was so good that I went and read the whole series, but I only remember the first book which she read. It was about a twin brother and sister in an adoption centre, no one wants to adopt them. Finally they get adopted by these two old ladies, who they later find out are giant cockroaches or something. They run away into the forest, in the forest a giant slug tries to eat them but the girl takes out her salt and pours it on the slug. After they go to this house and a normal guy is living with his wife who is like half spider half person or something, they want kids but she doesn't want to have to eat him. I think she tries to eat them or something. And at the end it said like" They knew one thing for sure, their adventures had just begun".