It may actually make more sense to imagine Oropher as a cousin of Celeborn - or more precisely, Oropher and Amdir Malgalad as first cousins of Celeborn and Galathil, sons of an unrecorded sibling of Galadhon. Let's lay out the evidence:
1. Amdir Malgalad was blond
The biggest pointer at this is that second name, 'golden tree'. Amdir came to Lorien before
mellryn grew there, so he can't be named after them; he's a Sinda, so he can't be named after Laurelin, and a Doriathrin Sinda, so Glingal of Gondolin is out too.
So why 'golden tree'? Well, Celeborn was originally intended to mean 'silver tree', and he's got silver hair. The same logic can be applied to Malgalad; the 'tree' part can be explained by Tolkien's description of Legolas as 'tall as a young tree' (and the fact that Thingol was the tallest of all elves is relevant here).
A second line of evidence comes through one of Amdir's people: Haldir, the marchwarden of Lorien. Like Thranduil, he is confirmed as having golden hair, but look at his name: it has that same
-dir ending. Tolkien translated an earlier use of the name as 'Hidden Hero', but it makes equal sense as a tribute to the late king: 'Amdir of the Shadows'. The most logical reason to name your child in honour of the king would be if he looks like him - say, if he had that same golden hair...
2. Since blond hair is rare among the Sindar, we can assume its bearers are related.
Tolkien went to great lengths to ensure that all silver hair descended from Thingol's family. He went to the same lengths to tie red hair solely to Mahtan's line. And every instance of gold hair in the House of Finwe has a Vanyarian origin. So why would things be any different when it came to the Sindar? If Amdir (see above) and Oropher (see Thranduil) are both blond, then it seems likely they were siblings.
(What does this say about Haldir, then? Um... pass.
)
3. Amdir and Oropher were royalty before they were kings.
I mentioned this already, but didn't realise the significance.
Unfinished Tales: The History of Galadriel and Celeborn: Appendix B is entitled 'The Sindarin Princes of the Silvan Elves'. But... Thranduil is the Elven
king. Amdir and Amroth are
kings of Lorien. Assuming that 'princes' goes back to Tolkien rather than being a Christopher turn of phrase (he uses it twice), it seems clear that Amdir and Oropher were of royal blood - which means they were tied in some way to the House of Thingol.
(Incidentally, it seems likely that Oropher was the elder of the two: it was he, and not Amdir, who let the Silvan host at Dagorlad.)
4. Their names tie them to Galadhon's line.
This is the original theory again: Oropher and Malgalad share 'tree-names' with Celeborn, Galathil, Nimloth, and Galadhon. All the evidence from the original post - Thranduil's halls, the Enchanted River - still applies here.
5. Cousins makes sense of the power dynamics.
It is pretty clear that Oropher didn't stand in a position of authority over Celeborn: if he did, he wouldn't have moved his entire kingdom north to get away from him. But equally, Celeborn doesn't have clear authority over Amdir: he entered Lorien as a guest, rather than accumulating power even by accident. So none of them fall easily into the 'elder brother' position. Equally, Oropher and Amdir can't be brothers of Galadhon - they would have almost parental authority over Celeborn, which they clearly don't.
But Elmo's grandchildren down a different line? A brother or sister of Galadhon who married a golden-haired elf?
That makes sense. It also avoids going against the non-narrative implication that Celeborn and Galathil are Galadhon's only children - there doesn't seem to be a similar implied claim that Elmo only had one son.
hS