I think that the "overnights" should be the exception, and sis should not expect this too often.

Take them for shorter periods, and as likem said, have something fun planned: A movie, an amusement park, maybe you can set up a PlayStation or X-Box at our house. Something that they want to come over for. If they get too wild, then no games. The good news is that they will mature in about 10 years, and will not get caught dead at their aunts house (unless they find your stash).