If that is passive what is its active form?
active voice: She sat on the chair.
passive voice: The chair was sat on by her.
That kind of passive is called (by many linguists) the
prepositional passive. In the active-voice sentence, you have the intransitive verb "sit" (in the past tense) modified by a prepositional phrase ("on the chair"). In the passive-voice sentence, the object of "on" is subject.
Alternatively, we could say that the object of "sat on" is subject in the passive-voice sentence. And if we said that, we might also say that "sat on" (two words: a verb and a preposition) is behaving as if it were one transitive verb in the sentence "The chair was sat on by her."
I have a book on the prepositional passive. I haven't read the whole thing, but I just consulted it, and "sit on" is one of the verb-plus-preposition collocations that work in the prepositional passive, provided the meaning is literal (not metaphorical) and that the location specified by "on" is affected.
Here are some sentences with "sit on" from the book. The red example with an asterisk and a line through it is ungrammatical. I am adding "active" and "passive" labels for clarity. Where two labels appear next to a sentence, the first applies to the first clause and the second to the second.
The hen sat on the egg until it hatched. (active - active)
The egg was sat on until it hatched. (passive - active)
The egg was sat on for three weeks. (passive)
John sat on my hat. (active)
My hat was sat on. (passive)
This bench shouldn't be sat on -- it's just been painted. (passive - passive)
Don't let yourself be sat on. (passive)
*[strike]
Three committees were sat on by John.[/strike] (passive)
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Couper-Kuhlen, E. (1979).
The prepositional passive in English: A semantic-syntactic analysis, with a lexicon of prepositional verbs. Tübingen: Max Niemeyer Verlag.