NO, it's that simple...no excuses needed. This is your life, do you REALLY want to share it with her???? Now THAT NO was easy, wasn't it??? So will the other one. This way, you'll have NO regrets!! See how that N-O is getting easier??? lol Stay strong!! It will give you more peace of mind and you've earned it. Keep thinking how happy you were when she left the last time and that you don't want to go through that again!!
(Plus your cats will HATE you for allowing that DOG in their space!! lol) :D