Keats says "Where youth grows pale, and spectre thin and dies" Is he explaining about 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.