You are right, fratres occasi is not correct. Occasus, -a, -um is indeed the perfect passive participle of occidere (with a short i), and this verb does indeed mean “to fall,” usually not in the literal sense, but in the sense “to perish, to be lost.”
However, since it is an intransitive verb, it forms no personal passive. Fratres occasi would mean “brothers that have been fallen,” and that makes no sense. The only exception to my knowledge is if the subject is sol, as in post solem occasum (after sunset), etc. But never mind that. Quod licet soli, non licet fratri.
But occidere has a doppelgänger, occīdere (with a long i), which means “to strike down, kill, slay.” It is transitive (unsurprisingly) and does form a personal passive, but the perfect participle is occīsum. So you could say: fratres occisi.