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I am assuming your sister must keep the dog outside, which in some circumstances is needed and can be done properly,and the dog is happy and healthy.
I agree with Victor, treat the area daily and keep dry. The cone should help him from continuing to break open the effected area. My daughter had the same issue with one of her goats a few years ago. She contacted a local farmer who had horses, he gave her a sauve to put on the ears of the goat. I'm not sure what it was called but it worked great. Once the area was healed the flies went away.
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