You have my sympathies! We had to deal with this when the girls were in grade school; our school district had it bad, and the lice was resistant to the pesticides. You can test for resistance pretty easily: take a live louse and stick it in a cap full of the treatment. If it's still moving 20 minutes later, it's resistant.
We learned that the most effective way to deal with the problem is frequent combing and removing of the nits by hand. There is just no other way around this. You do not have to do it every day, except initially. Do every day for a week, then every few days, then once a week. The nits are initially laid at the base of the hair follicle; as the follicle grows, you will begin to see them. This does NOT mean that there are still live lice on the head laying eggs; it just means that the follicles have grown enough you can see the nits that had been laid earlier. Any nit more than a half inch away from the scalp can be ignored as it's already hatched.
Olive oil can help - not only is it soothing to the scalp, and helps aid in removal, but you can leave it on overnight (wrap head in plastic wrap) and it may smother the live ones. I tested this out too: I left a live louse in a cap of olive oil overnight, and it was dead in the morning.
As for cleaning everything, you don't have to go too crazy with that. Do the obvious things, like washing sheets and pillowcases and hats. And vacuum everywhere, especially the backs of sofas. But understand that lice cannot last long without a human host, and they can't crawl either. So your head would have to be in contact with a nit at the exact moment the lice hatches in order to get re-infected.
As for stuffed animals and things that can't be laundered, bag them up and leave them in the car, in direct sunlight, for a few days. The heat in the car should be enough. But again, this is probably going overboard. A good shake out and then vacuuming of the rug should be just fine.
As for why you were spared, I have a personal theory on that (this is NOT scientific). Because I was also spared -- even though my kids often slept in the same bed with me. My theory is that if you blow dry your hair every day (as many adults do, but children don't), it blows the live lice out of the hair. If your hubby doesn't blow dry, this may be why he caught it, and you did not. So my other suggestion is that it cannot hurt to blow dry your girls' hair every day for the next two months.
As for how to find time to do all the nit picking, I would give the girls a bath, wash their hair, condition it with olive oil, then sit them on the sofa with their head in my lap, while they watched a movie. The girls found it relaxing. It wasn't ideal since by then the sun was down. So then on the weekends, we'd have "lice picking" parties with the neighbors, outside.
Which brings me to my last point: it's highly important that parents are open and honest about this issue. My kids are now in college, but that was a huge issue way back when -- parents were ashamed so they would say nothing, and kids would get re-infected. Lice has become such a problem in last decade, I am really glad to see that parents are far more open about it.
