Instead of hiding in your room, embrace your ownership of the apartment! Walk out with three pythons draped upon your person like a diva. When he introduces himself (and he should if he's a gentleman) just explain, "oh, I'd shake your hand, but I'm covered in rodent remains. Pardon me!" If the date doesn't get flighty and run, he's a keeper for your sister. If your sister gets embarrassed, kindly remind her that she won't have to worry about things like this happening if she had her own place to live. Heck, she wouldn't have to worry about it if she had just met her date at a restaurant or something.

In all seriousness though, consider drafting up a roommate agreement. Take some time to get your thoughts together on what is and isn't acceptable to you. Keep it reasonable (don't be petty- not that I think you would be) and go over it with her. Be open to suggestions, but firm in the areas you absolutely require from her. BOTH OF YOU sign it. Mail a copy to your parents, and if she continues to violate the agreement send her home. Put her on a reward system. If she plays with the cat every day for a week, does her chores without being asked, etc., she gets an hour of "company time" where she can have someone over (just an example- if you're not comfortable with anyone over don't offer it). Or, if she does something above and beyond the mandatory, say, picking up groceries one week, you'll take on a chore of hers for a weekend. If she submits X amount of job applications, treat her with a sister movie night. Eventually she'll associate work with good things instead of putting off doing it because it'll end in nagging (or what she perceives as nagging), fighting, and threats. It'll also give you guys a chance to build up your relationship and function as sisters for a while, instead of roommates. Good luck!