J.N.
I go through this with my inlaws...they refuse to go anywhere without their black lab (she is also a hunting dog). They live 4 hours from us in Springfield and I had to become the big "B" and tell them the dog is not welcome at our home. They have another dog which they kennel when they come up because he is so mean and I asked that they do the same with Molly or leave her with family members down there.
While I love dogs and have a 90 pound German Shepherd of my own....their dog is out of control around new people, is aggressive, and barks non stop....oh, and they do nothing to try and get her to stop.
I hate to say this but, because I allowed it for the first year or so it was much harder to get them to think how we feel. I told them I love them and want them to come visit but I could not take the dog and they would need to place her in a kennel or have someone watch her while they are here....NOW, while they don't come visit vey much anymore (nice huh) they do bring Molly but they stay at a hotel that allows dogs and they leave her there.
So anyway...my thoughts are
1)be honest w/ your sister about the dog (it might bite, shed,tear up new home/yard) and ask her not to bring him.
2)Place the dog in a kennel for the weekend or a doggie daycare during the day at least.
3)Let her bring him but be ready for the dog to come visit from here on out......
Sorry this is so long...major sore spot in my family as well....