I just read (here) that Vietnamese has a word for elder brother (anh) and elder sister (chi), but the same word for both younger brother and younger sister (em). This is the same as Malay, were elder brother is abang, elder sister is kaka, and younger sibling is adik, and no difference is made between younger brother and sister.
Of course, there is no suggestion that Malay and Vietnamese are connected, as they belong to entirely different language families. But I wonder if it might be an aerial feature, a feature of language that extends across language families because of proximity?
Or maybe it is just a coincidence.