This is a pretty unique brand of incest-less incest, I will grant... but it's still really uncomfortable at bits.
The kid obviously sort of still thinks of the thing as his brother, keeping this all firmly in the territory of deliberate incest erotica, just with better-than-average guilt-alleviation measures taken—for those who did want to read such a thing but felt bad about it, I should say. There's really no plot nor emotional necessity to the brother angle beyond that, that I can see.
Well, I guess I'm just stating the obvious here. Generally I'd have not read this far enough into incest-y stuff to comment on it, but high-fantasy romance and the like is one of my greatest weaknesses... so here I am.