why not 'tui fratres dormiunt'?
It's also possible. It's more common with "fratres tui", as the possessive usually follows the noun (according to the Wikipedia page about Latin word order), but it's not wrong.
Also, your version is more emphatic on the "tui".
Should there really be capital letters in latin sentences? I heard somewhere that latin does not natively have capital letters. Are they there for aesthetics and hints?