Stupidity: Claire, in particular, and her husband are fascinated with Mrs. Mott and immediately accept her as a nanny, without looking into her background to validate her claims of previously being a nanny, or to learn more about her personal job history.
Stupidity: When Marlene deduces "Peyton" as being Dr. Mott's widow, she drives to the house to warn Claire. When she arrives, she foolishly decides to confront Mrs. Mott with this knowledge right to her face and demands to know where Claire is... which ends up with her being killed in Claire's place. Instead, she simply should have waited or looked for Claire, rather than confronting "Peyton."
Answer: The same way any common criminal gets in to a house, she "broke" in by either kicking the door, or smashing a window, etc. A locked door is not that hard an obstacle to overcome.
Jazetopher