It does not take a genius to observe that Marui sisters are named sequentially. But what's puzzling, they are also given numbers, and the numbers run in the opposite direction to names. For example Hitoha is #3 (see the screencap).
UPDATE: Just saw this in the preview for ep.3:
I never encountered these characters and they do not look like normal kanji numerals. But undoubtedly they are the numbers we just discussed. Most likely they are Roman numerals (Ⅰ Ⅱ Ⅲ - in some fonts they look just like the print at swimsuit labels, even with the top bar shorter than the bottom one).