According to Plutarch*, Themistocles had at least one brother. Supposedly his name was Agesilaos and he was a scout sent by the Athenians to spy on the Persians.
While it's difficult to imagine that Themistocles was an only child, many ancient authors put a lot of stress on his foreign blood. It may be that any siblings he had were not actually full kin. They were either related by blood through his father, or not related at all (step-siblings).
In my novel Themistocles does indeed have a step-brother, and a wicked one to boot! Here's a small sample from my novel introducing the Bully of Phrearrhioi.
The Bully of Phrearrhioi
The morning dawned warm and fragrant, as it always did in summer. But it was not the heavy scent of thyme wafting down the slopes of Mt. Hymettus that stole me from Sleep's embrace. It was a fierce kick directed at my face.
"Wake up, foreigner! Neocles wants to leave for the festival before it gets too hot." Rubbing my temple, I shrugged off my wool blanket and sat up. "Good morning, Sir Piggy," I yawned. That earned me another kick, harder this time and better aimed. For a moment I saw stars. "I told you. Don't call me that! I'm better than you are. I'm a citizen of Athens and you're just some stupid butter-eater! So you show me some respect." "I called you Sir**," I pointed out, making sure to put the bed between us as I climbed out of it. My step-brother glared at me through small, squinty eyes half-hidden in folds of fat. I was not family to him. I was merely some Thracian's get that he had to put up with because my father had married his mother.
"I don't know why you're going. Foreigners shouldn't be allowed to attend the Panathenaea. But then, maybe your father finally decided to sell you to a slaver in the Agora!" He squealed a laugh. I addressed him levelly as I attempted to smooth out the wrinkles from my favorite tunic, a square of faded green cloth with a blue border. I wore it everywhere, even to bed. "If my Da wanted to be rid of me he would have put me in a pot and left me out on the street like all the other babies nobody wants. Your parents tried that but you were too fat to fit." Agesilaos turned pink, only making him look more the pig. "I'll beat your ugly face in!" he hollered, shaking a hammy fist at me.
"Is everything all right, young Master?" Agesilaos' paidagogos appeared in the doorway, a look of concern on his face. At 80, Xenos was thin as a reed, as blind as the Fates and as old as Chronos. Agesialos speared him with a look. "Mind your business, you old fool! And where are my things? I told you to lay out my best cloak and sandals! My mother says I must look my best today. Oh. And get something for the butter-eater to wear, too. I don't want him embarrassing me with that UGLY tunic." "Alright, Ages. I shall do so as soon as I can." "Do it now!" Sir Piggy crossed the room and gave him a vicious swat. The old man yelped and scurried away. Agesilaos laughed. I glared at his back, dreaming of the day when I could smash his ugly snout in. That that was far away, though. I was seven to his twelve, and could not hope to best him in a contest of strength. And so I endured...
* Plutarch, Moralia.
** The word "sir" may strike readers as anachronistic, but there was indeed a sort of class system in Ancient Athens. Nobles were the top tier, the knights came next, then the middle class and finally the poor. Themistocles would have been ranked as middle class, and his step-brother --through his mother's side-- from the knights or noble class.