Chance are that Tig Tig is hiding nearby someplace. The best thing is to go out late at night with a good flashlight and check under every bush, porch etc. around the area. Also bring some catnip and put it someplace where it can be watched from nearby (even in her car). Leave food & water nearby but not when it gets dark out. That may attract in preditors. Purrayers that your sister's kitty will be safely indoors again. Keep us posted.