Sorry for the delay in responding, but it looks like I have an idea that nobody else recommended. Have you ever considered bed bugs? I know is sounds gross, but my friend had a reaction that the doctor thought was allergic hives. It was 2 months later that she discovered bed bugs in her bed! The way she found out about it was when she changed her sheets, there were small red blood stains on them. Bed bugs can be small (like the size of half of your pinky fingernail) and usually dark red in color (from the blood they suck out of you!) They are attracted to your carbon dioxide so they generally don't come out until it is dark, and you have been lying in the bed for about 20 minutes. They nest in wood. You can take a spray bottle of mostly rubbing alcohol (mix 10:1 alcohol to water) and spray the wooden areas around the headboard/footboard, etc. and watch closely. They will try to get away from the alcohol and you can then see them.
I hope this is not your sister's problem, because it is very expensive to get rid of them. And, since he came home from the hunting trip with them, it doesn't sound like a conncetion. But, if he seems to especially have a problem at night, maybe it's worth checking.
By the way, these bugs are picked up at hotels and it seems to be becoming a bigger and bigger issue (even, and especially at expensive hotels!). Now I always travel with a spray bottle filled with alcohol to check before I stay in the room!
Hope you find a cure!
K.