I would suggest placing fly traps around the area of the dog. Here are some homemade biting fly trap ideas. It mentions that they are for biting flies of horses, but believe me a fly has no care whether it's biting a horse or a dog and I'm sure the species of flies that feed on horses are also related to the flies that are biting your sister's dog. http://www.showhorsepromotions.com/horseflies.htm
