A letter to a brother may include advice, news and reminiscence about shared memories. Depending on the purpose of the letter, it may also include apologies, condolences or an expression of congratulations.
A letter to a brother should include greetings, news of the letter writer's life and questions asking about the recipient's life. Since a brother is a close relative, the letter may also include funny or sentimental stories from the past.
The exact content of the letter depends on its purpose. If the writer has recently angered her brother, the letter may include an apology for poor behavior and a request for forgiveness. If the brother has recently been hired or promoted, the letter may express congratulations. On the other hand, if the brother is suffering from a difficult time in his life, the letter may express sympathy and offer advice.