In traditional Balinese culture, it was common for a set of twins of the opposite sex to marry each other, since it was assumed that they'd had sex in utero.
@toshiNARI: On page 7, the reason it's in katakana is because she's trying to say it in English and her pronunciation is terrible. You should use a different font to indicate the word is "Engrish" and change the translation note to make that clear what's going on for people new to manga (Since they won't know what katakana is).
@StrangeStripes I precisely spelt little brother in Engrish to make a point that she was speaking in botched English, but thanks for the different font tip. As for the TLN... I'll just add tack a brief explanation onto the end card cuase of space reasons.