First of all, let's establish the subject of the subordinate clause:
I would like to know what it is.
*I would like to know what they is.
*I would like to know what it are.
I would like to know what they are.
It's clear that the verb agrees not with the first nominal ("what") but with the second ("it" or "they"), making the latter the subject. (This word order is normal for such clauses.)
In your sentences, the subject is "the rest of the sentences". Unfortunately, many dictionaries don't do a good job of explaining that the noun "rest" can be either singular or plural with no change in spelling. Nevertheless, the American Heritage Dictionary correctly suggests that either is possible:
That or those remaining: The beginning was boring, but the rest was interesting. The rest are arriving later.
In this regard, "rest" is similar to indefinite pronouns such as "all" and "some". (In fact, the Collins Dictionary even claims, "Rest is also a pronoun". I wouldn't agree, but the claim is interesting.)
Beecause in your sentences "sentences" is plural, "rest" should also be construed as plural, so the verb should be "are".
(Edit: It might be possible to construe "rest" as singular if it actually refers to a single quantity, even of multiple sentences. However, using "is" in this situation is liable to sound awkward, so I recommend against it unless you really must denote the singular meaning.)