I have used two rules for pronouncing words and names with double vowels. The first I learned in elementary school: when two vowels go walking the first does the talking. The second rule is for German words and names: paraphrasing rule one: when two vowels appear in German, the second does the talking. Exceptions to both rules are diphthongs, or two vowels pronounced as one vowel sound, e.g., noisy. And while we're at it, Hawaiian words are often mispronounced. Kaua'i is not pronounced Kuh-WHY. It's kuh-WAH-ee.
There is no consistent rule in the English language stating how to pronounce vowels, whether singular or doubled.
Neither is a particularly awkward case, since there are two common pronunciations of the same word: naɪðə or niðə. In the former, the "ei" is pronounced like the word "eye", while in the latter, it is a long "ee" sound, as in the word "weed".