It's a quirk of the Wade-Giles romanization system. In Pinyin, it would be rendered "Yi" (pronounced "yee"). "I Ching" in Wade-Giles is "Yi Jing" in Pinyin, for instance.
Sort of extending beyond that, if you put all the Monarchs' names into Pinyin, they would be Ming Yi, Bi Dui, Huan Gen, and Li Ding. Obviously that's getting a bit deep in the weeds for Feng Shui, but I'm a language nerd and the thread got me wondering.
Relatedly, there's an inconsistency in the Feng Shui core book that's common to both editions. In the introduction, when it summarizes the factions, it gives "Pi Tui" as the Queen of the Ice Pagoda, but in the chapter on the factions in detail, it gives "Pui Ti." I think this also crops up in some other first edition material outside the core book (I don't have my full FS library to hand, alas). I stick with "Pi Tui" because "pui" isn't a syllable that occurs in Mandarin Chinese (can't speak definitively to the other dialects).
P.S. Their names read like given names (two character given names are common), assuming they aren't some type of alias. Presumably, being siblings, they all share a family name. I don't think there's a canonical answer to what that is, so there's some room to tie them in with characters as distant ancestors/cousins.