I wait until the last day (or the day before). I don't throw everything in, I pick and choose, and I'm not even so efficient at this. It takes me 2 or 3 hours to finish packing, but my sister finishes packing quicker and more efficiently with two kids (she learned how from her husband though, lol).
Unless I am traveling last minute, it stresses me out too much to wait to the last minute. But like your sister, I could pack for myself and all 3 kids faster than my husband could pack his, and ultimately he would forget something. It finally got to where it was easier to just pack for him too! Thanks for the input!