My friend, believe me, it is difficult to live in a tropical country and not have tan marks, for example, my left arm is darker than my right, just because I drive, the arm on the window suffers a lot from the UV rays, and I don't even have to drive that much.
So it doesn't seem realistic to me that these girls don't have them living on a tropical island, even if you use a lot of sunscreen.
The only one who wouldn't have tan lines is the vampire in the Halloween special, because she would literally burn when exposed to the sun.