I'd say your wife's fears are generally unfounded, but common sense is in order.
Snakes... are just emerging about that time of year, starting at lower elevations. Your chances of encountering one in Prescott Valley are low. We were setting up camp near Tucson in early April when a rattlesnake crawled lethargically across our site and right under the trailer. It's belly was bulging with a recent meal, and our arrival had likely disturbed its post-meal slumber. It slowly (and silently- apparently it didn't consider us a threat) made its way into the brush and went back to sleep, or so I guess- we never saw it again. We kept our distance, and there was nothing particularly dangerous or frightening about it.
Scorpions... are more of a nuisance and also more active in warmer months. They like to hide under and inside pillows, blankets, clothing, and shoes, so shake out anything left lying around outside (or better, don't leave that kind of stuff lying around in the first place). We've never had one inside the trailer- I give attention to making sure the door seal has no gaps all the way around- but I can't say the same for our house. They are fairly regular visitors, mostly in July and August, but occasionally all year round. In general, limiting clutter gives them fewer places to hide and makes them easier to spot. Once spotted, they freeze, so they're easy to kill. Our house is in an area of rocky, volcanic ground where they like to build underground nests. Your brother can tell you how big a problem they are locally.
If the worst happens, the sting of most species is like a moderately severe bee sting, usually accompanied by some minor numbness, lingering for several hours- ice and benadryl help. We've only had 2 stings among 4 people over 12 years, neither requiring medical attention. It's my understanding the bark scorpion can be more dangerous. I haven't encountered one. Your brother can also tell you whether there are any scorpions in his area you should be particularly concerned about.
Relax and enjoy your visit!