Your not going to get that by debating her. Your going to get that by not debating her. Just the fact that you wont offer her the opportunity to gain an inch is enough.
You must be strong but supple. In her mind your the one being stubborn. See to her she has all the great reason to not keep them and she'll work just as hard to convince you of her side as you do of yours.
If that chance never comes then neither side has to concede.. She may never except your snakes are you willing to spend your life trying to change her mind or are you willing to except that she doesn't and move on?