Are they going long distance, or local? Either way, a sturdy carrier is a must, along with collars with not only the old home address, but the new one as well in case one gets loose somehow. Keep the cats in a room where nobody will enter while things are being moved out of the home (tape across the door saying DON'T ENTER)! Once things are moved into the new house, keep the cats in one room with familiar things for them such as beds, kitty litter boxes, anything that smells like the old home, and will keep them secure feeling. Maybe a good idea to tape across the door if things are being moved in still, to avoid door darters! Once things have settled, allow the cats out to explore the new place, and expect that they may or may not hide for a day or so. Be sure screens are secure also so they don't rip them open and escape...I've heard that happens. Good luck to your sister-in-law to be, and I'm sure her kitties do just fine.