Demeter has beaten me to it while I was working on my own much more literal translation. Here it is anyway.
His brother died about 57 BCE in Bithynia. Catullus visited his grave in company with C. Memmius Gemellus and possibly his friend Gaius Cinna.
Travelling over many countries and many seas
I arrive at these, brother, poor offerings to the dead,
that I may finally give my duty to the dead,
and much for nothing address your ashes,
seeing that Fortune tore you yourself away from me.
Alas, sad brother unworthily taken from me.
However, now in the meantime, in the old-fashioned custom of our parents,
these sad duties are handed over as offering,
accept them with much flowing of fraternal tears
and in perpetuity, brother, hail and farewell.
(Later) For much better translations than mine see also Catullus 101.
Thanks to bertilak for identifying the word manantia.