As a different take, I decided to draw this out as a family tree, taking the first member of each line to be the same as the second of the preceding line. We get a pair of male/female siblings marrying a pair of male/female cousins - a closed chain, if you will. Now, do I really have to go perusing through the genealogies to find this pattern?
Hmmm...
Hurin/Huor and Rian/Morwen are siblings marrying cousins, but they are not male/female pairs.
Hareth/Haldir/Galdor/Gloredhel are male/female pairs, but I think both pairs are siblibgs, not cousins.
Darn it! Fiddling with bits of genealogy, I can only make pieces fit, but not the whole thing. Urwen, you tend to be better at this stuff than I am. Can you spot the pattern?
Now your hint about Not Hobbits makes a lot of sense.
