Anyone who thinks older sisters protect younger ones has never heard me tell about my sister Pam. There's no great in our ages-Pam is only three years older- but throughout our childhood she was always able to beat me at cards, at jacks, at all board games.

The answer should be written as : "there is no great difference in our ages..."

Explanation:

As the speaker was talking about the differences in the ages between her and her elder sister, then the correct word should be difference. She mentioned already in the context that her sister was only 3 tears older and was supposed to protect her but she tended to beat her in all games during their childhood. She actually expected the sister to let her with some wins and not humiliate her as such during the times they played games in childhood.

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