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.
"aanhaa" here represents "brothers", so you also need "brothers" instead of "brother".
I hope it helps.