Because Kim never seems to just take a vacation without some sort of work involved, she was in the spa area doing a sexy photo shoot while the rest of the fam was relaxing around the compound.
For some reason, Brody didn't get the memo and he walked out there while Kim was doing her thing in a sheer white outfit. Kim quickly tried to cover up, but the damage had already been done.
When Brody walked into the other room, he told his brother and sister-in-law about what had happened and it came out that he MAY have been a little turned on by his half-naked stepsister, which is sorta weird.
The thing that surprises us the most is that this appeared to be the first time he had seen Kim naked cause we've pretty much seen Kim in various states of undress, whether we've intended to or not. It just sort of comes with the territory when you have an internet connection.