Clip one wing on each bird and that should keep them from going over a 4' fence. Akitas were bred to hunt bears, they are strongly prey driven. Obviously you must be the one to take responsibility for the situation. Can YOU put the dog on the chain rather than asking someone else to do it? Also, let your sister know what your birds are worth in dollars and cents. Tell her she will be expected to reimburse you for (a) medical bills, vet fees, for injured birds, (and call your vet ahead of time so you can let her know the base price for a vet visit); and/or (b) the cost of replacing dead birds her dog kills. For my birds, which are nothing special, I recently charged my neighbor $25, about the cost to replace a started pullet. At that, I did not include the cost of shipping had I actually decided to replace them. That would likely have doubled the cost. Once she knows her irresponsibility is going to hit her in the pocketbook, she may take the situation more seriously.