powdered gelatin in the shower head trick?
That, or the beef bullion cube trick in the shower head. Something else fun would be to replace family pictures with those of random people (INGOERs) and see how long it takes them to find out.
This would be pretty cool.. we could host an INGO photo shoot and try to match the poses as best we could, too.
Wow...Film a very low budget adult film in their house and leave it as a welcome home present. since it's your sister, I would suggest that you not play a starring role in the project.
Get some actors to dress as a homeless family and move them in about an hour before they get hack. Make it look like they've been there for a week.