Thanks for bringing this up, made me go through some grammar books. On page 43 of this one: http://blaustein.eps.jhu.edu/~afuller/books/Persian%20Grammer.pdf The concept of "collectives" is discussed. It's using a word in it's general sense. Not definite, and not indefinite form of it. For example, a tree (indefinite), the tree (definite) and trees (the general idea of the thing). In Farsi, when you want to use a noun in its general form, you don't use it as a plural, but both the pronoun and the verb you use should be plural. So in اینها درخت هستند. /Derakht/ which is the complement of the verb, is in singular form, just because it's tree in it's generic sense. But all in all, my first answer was oversimplified, but it's correct anyway. I suggest reading the grammar book that I gave you its link, and reading Farsi texts until you manage to use the structures without thinking. We were a couple of Persian speakers in the room when I saw your question, and we both knew number one is wrong, but it takes a long time to explain why :)

2. is correct. They are my brothers. In Persian the number of persons who do something, effects the form the verb you use. i.e. if one person does something, or more than one person do it. "They" (aanhaa) refers to more than one person, and it requires a plural verb in Persian. آنها هستند. "aanhaa" here represents "brothers", so you also need "brothers" instead of "brother". I hope it helps.

Dear Shirly, Second one is true And why is the fist one is incorrect? Pay attention to translate of two statements in English below, that may help you: 1- They are my brother. 2-They are my brothers. They" and "are" will use for a plural name!!!!!! Brother= برادر (singular name) Brothers=برادران (plural name) So that is why brother is a singular name we should use singular verb and Pronoun for it and brothers is a plural name so ,plural verb is correct. The correct from of first statement is: آن برادر من است.