Silm fandom is probably the least concerned fandom of all (that I know of) when it comes to such relationships; but I’ve seen TONS of fanworks where both Maedhros and Fingon are judged by their families for their “incest” or something. Or not judged, but somebody still comments on it. But would it had been a problem in canon?
We have some examples of cousins in love. Or at least one-sided love. Like Maeglin and Idril.
Cousins. Too close to marry. “The Eldar wedded not with kin so near“. That’s a problem. Okay.
We also know that in Tolkien’s later writings Celeborn was not from Doriath, but a Telerin Elf, grandson of Olwё. He still married Galadriel, tho, and left Aman with her.
Aaaand they’re cousins. Apparently, that’s not a problem now. So what do “Laws and Customs” say about close kin relationships?
…okay then. So cousins CAN marry (like Galadriel and Celeborn); and I suppose the biggest problem in case of Idril and Maeglin was Idril’s strong dislike of the latter. And Maedhros and Fingon are not even first cousins, they’re… half-cousins? Second cousins? Idk how this mess is called.
We also have another example, also with Fёanor’ and Fingolfin’s children. It is said that Aredhel “was often in the company of the sons of Fëanor, her kin; but to none was her heart’s love given.“ Like. Maybe my English is too bad, but to me this phrase looks like she could have a relationship with one of them, but didn’t want to.
So what I’ve learned today is that while people on Tumblr have a really harsh policy towards incest, Elves didn’t give too much shit. Also Russingon is canon now, I suppose.