According to Dictionary.com Menelaus's brother was Agamemnon..
According to my book, Gods, Demigods & Demons, An Encyclopedia of Greek Mythology by Bernard Evslin, Menelaus… is the brother of Agamemnon..
In my book, Mythology - Myths, Legends and Fantasies by Global Book Publishing , Menelaus is the son of Atreus and Aerope, husband of Helen and brother of Agamemnon who was married to Clytaemnestra. Menelaus had one daughter called Hermione..
Menelaus was the king of Sparta, while Agamemnon was the king of Mycenae..
Hope this helps!.