You cannot use an object pronoun after a linking verb; you have to have a subject pronoun because it's the same thing as the subject (noun or pronoun).
This = N1s = Subject Pronoun (s = noun substitute, i.e., pronoun)
is = LV= Linking verb Connects a predicate noun, predicate pronoun, or precidate adjective back to what is the same as the subject noun or pronoun or describes the subject noun or pronoun
she = Predicate pronoun
You are dealing with the N1s-LV-N1s (subject noun substitute/linking verb/ subject noun substitute) sentence pattern. You cannot say you are her (object pronoun) because you are the same person as the subject "this".
It is he whom I am speaking to.
I am he. I am Joe. Joe is I.
He is the paperboy. The paperboy is he. (You wouldn't say, "Him is the paperboy,'' so how could you say, " The paperboy is him."
Compound subjects with linking verbs:
It is Betty and Sue. It is they. They are Betty and sue.
Predicate nouns
N1-LV-N1 Bob is the manager. The manager is Bob.
Predicate pronouns
N1s-LV-N1 He is the manager. N1-LV-N1s The manager is he.
N1s-LV-N1s He is it. It is he.
Predicate adjectives
N1-LV-Adjective Joe is handsome.
N1s-LV-Adjective He is handsome.