Sadly, DH has no family, so no options there.
As for the conference...if this were required for work, that would be one thing, but he goes to a ton of conferences, at least one a month. He only wants to go to this one because it's in Las Vegas.
And while I agree that sometimes you have to do these things for work, our employers both understand that we have to make trade-offs sometimes. In the old days, if a man had to be at a conference, there would be no question that it was the wife's job to stay home with the kids. Nowadays, DH and I are constantly juggling our travel plans so we don't have to be gone at the same time. Our bosses don't hold it against us, because they have to do the same thing with their spouses.
ETA: I added this reply before I read the rest of the thread, and I agree with the responses to the poster who felt I was being selfish. Of course I'm being selfish, taking my folks up on a trip without my DH and DD. That's causing a bit of family drama in itself.
I will suggest that DH look into the possibility of childcare in Vegas. I don't think he looked very hard.
I kind of feel for your husband. He is likely getting pressure from his boss and it might not look good for him to say no. It's not the boss's problem either and many jobs do require these conferences and include going to them in evaluations for raises and promotions. Your husband is likely in a tough spot here.
As for advice, I would consider asking if she can stay with a school friend, or even taking her out of school and sending her to his parents for the time you are gone. Also College aged kids might all be in school now, but what about asking one anyhow. Their classes are likely around the time she'll be in school , anyhow - and even if there is a little bit of an overlay, she's 13 and could probably handle a little bit of time alone til someone got home. You might even look into getting a Grad student who would be a bit older than the average undergrad. Just a thought.