For one of the two: She is the twin.
adjective?: The twin brothers.
Two of them: They are the twins.
To describe a story:
use present tense.
only use past tense when something happed in past: they were human and be tramformed to vampires.
Always describe something you mention in paragraph when it is the first time you mention it. Even you already describe it in the last paragraph.
Just show your problem and what you need.
"can not" is more formal than "can't"
For a letter to professior, always can't be to too cautious.
If you don't know the gender of professior, Professior (whole name or last name) is a good choice.