Which part of the question is the dominant clause?

‘‘Tis is a very colloquial form.”. It’s prime use is casual. It is a direct question, asked in a way that lets the person you are talking to know that you thing the answer is ‘yes’.

There are several ways of asking a question of this type. One famous one used as a slogan for the anti-slavery movement in the nineteenth century.

Am I not a man and a brother?

More informally you can use the same curious shift from the positive “I am” to the question form followed by ‘not’. In more ordinary conversation You might say:

Wasn’t that a Ferrari that just passed us?

Another form of question starts with the positive assertion:

You’re Cox, the famous TV scientist, aren’t you?

Your example is most similar to this one but is still distinct. Sit is a perfectly good sentence. There is no ‘dominant’ (or ‘main’ clause). Or rather, the whole sentence is the main clause.