Now behave yourselves people.
It is not uncommon for royalty and nobility to marry second cousins. No one else but their own family is good enough, apparently. If you look at the British royal family, going back a few generations, it is surprising how many couples were already distantly related. I am not trying to deride the royals, it was just common practice a while back.
Elrond and Celebrian were also distantly related (via Galadriel's family), as were Elwing and Earendil.
However, Feanor's 'attraction' to Galadriel seems to have been considered innapropriate, and Maeglin's desire for Idril (first cousins) was considered incestuous. Even elves appear to have had resrictions on whom they could marry.