Naina thinks every compound has one word describing the other, as in "neelkamal"

Why

Naina is applying the pattern from 'नीलकमल' everywhere, the same overreach seen elsewhere in this chapter. Hindi has multiple kinds of compound words, and recognising which kind a word is depends on that specific word's own meaning, not one universal rule. 'भाई-बहन' is like 'माता-पिता' — both words are equal, neither describing the other; it shows two separate, equal people, 'भाई और बहन' (brother and sister). Test it: try reading it as 'a sister like a brother' — that meaning sounds forced, while 'brother and sister' is the natural, correct meaning. Her friend is right: 'भाई-बहन' is द्वंद्व समास (dvandva) — 'भाई और बहन' — not every compound works like कर्मधारय (karmadharaya); each word has to be recognised individually.