Hi JMLBrats. The thing about your post that worries me is that you state your sister (SIL?) has tried shampooing at least twice, plus a mayo wrap, and nothing seemed to work.
DS has brought home head lice not once, not twice, but four times! Twice it was limited to him, once he gave it to me, and once he gave it to me and my DD. Although DD was younger at that time, her hair was still probably about 30 inches long, so that was pretty nasty.
It helps if you're like me, and completely neurotic about things like that. I would lie awake at night, unable to sleep, and have lice nightmares. It might not be a great way to live, but it's a wonderful philosophy for getting rid of head lice. In my experience, the easy-going families sometimes had lice issues for weeks and weeks.
In each case I was able to get everything under control within 2 days, and completely eradicated in under a week. I'm not happy about using chemical shampoos, especially those like Quellada (sp?), and unfortunately, many strains of lice are now resistant to removal by chemical means. So, we used Dippety-do (a hair-styling gel), slathered on and kept under a shower cap overnight. It's a treatment recommended at our school. HOWEVER, it kills some of the lice, and slows down the rest, and whatever option you use, YOU HAVE TO NITPICK LIKE A MANIAC with a licemeister comb, too. My nitpicking schedule was twice a day for the first week, and DD's hair alone took about 2 - 3 hours to nitpick each time.
I washed every comb, brush, towel, sheet and pair of pyjamas every day. I also washed every item of clothing worn every day (I normally would do that anyway, but that included hats and jackets in the winter). I cleaned all the car seats, and put a towel (removed each day) on the headrests if we needed to use the car.
I wouldn't wish head lice on my worst enemy! Lice eradication became my full-time job for a week, and I continued with shorter follow-up checks for many weeks after.
I really like jchick's idea of a "certificate". Anybody who is currently going through this should understand completely why someone else wouldn't want to get involved.
In terms of waiting..... they can last for weeks and months if not completely eradicated, especially if the original source (perhaps school? daycare? camp?) isn't cleaned up as well, which could result in the family being reinfected.
Yuck! Yuck!! Yuck!!! You don't want to go there!
Best of Luck!