IMO, another reason Catherine is taken in by the Thorpes is because they meet her with such seeming warmth and friendliness, and since she is used to taking people at face value, this just seems to prove what lovely people they are (at least at first). As you say, if you're used to a family who loves you, but is prone to being less-than-flatteringly honest about you, it's very pleasant to meet people who tell you right out that you're terrific!