Factual error: When Marlene goes to the greenhouse and is killed by the falling glass, this would never have happened. It is a federal law that all greenhouses, skylights, and entry-ways are to be made of safety glass. The glass would have to have been made of either a Tempered glass, which breaks in a thousand pieces, like a car window, or Laminated glass, which cracks but does not fall apart, like a car windshield. Neither one of these glasses would have killed her. Surely, law abiding citizens like Clare and her Husband would have followed the zoning laws that came along with building their greenhouse.
Factual error: The glass shattering and falling in the greenhouse would have been unlikely to kill Marlene, let alone even cause that much damage to her, as the glass would have very little to no time to gain more force, falling at that low of a height. And if Marlene were to be killed, she would have probably bled out over time, as opposed to almost instantly dying as the film implies.
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