I've heard about alot of problems arising on the first day. This happened to us as well. Kaedyn attacked Kai at every opportunity. We highly doubted we'd be able to keep Kaedyn but things smoothed out on their own within the first week (they still weren't perfect but he wasn't attacking him anymore). Use your judgement. If you think it's possible that she may be able to stay, give it time. If you think it's too risky, then by all means, take her back. It's much better for the dog and for yourselves to take the dog back if there's no possible way that you would all be happy.