Is "niece and nephew" preferred over "nephew and niece", or vice versa?
I tried using Google NGrams, but it gave inconclusive results: one was more common before 1980, then the other form became more common.
I searched for the plural equivalent of niece and nephew: Google Ngram shows that nephews and nieces used to be the most popular sequence but since the 1980s that dramatic divergence has narrowed considerably.
From the 1800s until the mid 1980s, the dominant order for the singular form used to be nephew and niece, but the gap has decreased steadily ever since, and around the 1990s, the order niece and nephew has became marginally more popular.
All of which suggest that the binomial order is no longer irreversible and today the order of preference is totally arbitrary.