Yeah, I figured it was a malaria thing. So far as I know, the only human 'adaptation' to keep us safe from malaria is sickle cell anemia in north africans, the parasite can't infect the bent up red blood cells.
There are pills that are supposed to help prevent malaria, I don't know how effective they are or how long in advance you have to start taking them. I know an old friend of mine got malaria while on the pills, but he failed in one of the main requirements: taking the pills!
IME, clothing coverage is the only 100% way to keep them off, and even then a little spray is helpful since mosquitoes will bite through thin cloth. Also it seems like repellants have a fairly limited range and the bugs do seem quite adept at fininding missed spots.
The clothing sounds like something I've done, soaking clothes in repellant, but if she's wearing shorts/t-shirt, she'll still need to spray repellent on exposed skin.
On a slightly off topic note, I'm just a little jealous of your sister, that sounds like it would be a great hike.