adverbs - Can a preposition have the form of superlative?

They had almost reached the door when a voice spoke from the chair
nearest them, "I can't believe you're going to do this.”

I guess nearest is at the place of preposition. Can a preposition have the form of superlative?

Answer

Near is a bit of an unusual 'frozen' word. It was originally the comparative form of nigh (from OE nior). The terms nearer and nearest came later as speakers reinterpreted near as a positive form.

In addition near is rather vague with respect to its word class. The OED Online at near, adv.2 (and prep.2) notes the "difficulty of distinguishing the adjectival, adverbial, and prepositional or quasi-prepositional uses" of the word.

Regarding the comparative and superlative "prepositional" uses, the OED (ibid.) says:

When the noun or noun phrase is the direct complement of near , this
acquires practically the force of a preposition, but differs from true
prepositions in having comparative and superlative forms.

...(emphasis mine) which is exactly the case here. In short, it looks like a preposition, but it's a sneaky adverb.

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Also, at least in etymological origin, the to in "near/-er/-est to them" was actually a later addition to the idiom from Middle English, so it's not well-motivated to regard "near them" as a to-deletion as opposed to retention of an earlier form.

Old Icelandic nær (like Old English nēar ) might be used either alone
or with a noun complement in the dative case. Both usages were adopted
in Middle English, and a further construction introduced by the use
of to before the noun.