.. How do I get the peridot?

After Penelope tells you that Madison stole her peridot, go back and ask Madison about it.

How can I find out more from Penelope?

Naturally, she refuses to let you have it. She then asks you to leave because she's meeting her husband for dinner. Ah, so the place will be empty, right? So you're set --

-- as soon as you get the key to the apartment they gave to Penelope, that is.

How do I get Penelope's key?

Once you have the key, go back to the Haines apartment and let yourself in. Go into Penelope's room and take the peridot, which is hanging on the bedpost.

And suddenly, the Haines family is home again. Damn.

How do I get out of the apartment unseen?