Keats says "Where youth grows pale, and spectre thin and dies" Is he explaining about his brother?
There is a definite reference to his brother. I think that at the time of writing the poem, Keats was already profoundly occupied with his brother's predicament and wrestling with the issue of death, in general. However, another read on the poem could be involved in the entire stanza, where he discusses how the issue of age is something that plagues mortals. The third stanza opens with an allusion to a world where death and issues connected to mortality do not exist. His transition from this elusive world into the world in which we, including he, exist is where he talks about the withering of youth, or the "youth grows pale." Perhaps, Keats is making a statement on the cursed condition of growing old, something that impacts everyone. This reading makes the poem more of an elegy to youth, impacting all mortals, as opposed to something that is only intrinsic to him and the passing of his brother.
Yes, you can read this line from Keats's 'Ode to a Nightingale' as an autobiographical reference, a sad reminiscence of the poet's brother & his tragic untimely death. In fact, you can also believe this line to be a prognosis of Keats's own consumptive decay & death.
But you can examine the constructional and semantic value of the line to find its more general/ universal character:
a) 'where' which occurs in the initial position of this and successive lines in the form of an 'anaphora' repeatedly refers to a place--the lived life full of sufferings, bound by mortality;
b)'youth' which is an abstract idea here represents young men/women; this is an example of the use of the abstract for the concrete and therefore a case of the figure called 'synecdoche';
c) 'youth' which is an abstract idea gets personified in grows pale and dies;
d) there is an analogy in spectre-thin which alludes to the decaying body due to consumption; the word 'pale' also suggests consumptive decay;
e) the line shows a climactic, ascending pattern, describing the gradual process of decay and death, held in a universal cause-effect relationship.
Romantic poetry is generally highly personal and subjective, but all great poetry takes off from the particular to the universal.