Were Oropher's line related to Elu Thingol?

[Tyelkormo the Fair]

I've seen quite different opinions regarding this topic,some say that he was simply a nobleman of Thingol's court while others think he is somehow related to him through one of his brothers.
I don't actually remember the reasons supporting the latter but here are some of them

1. Legolas saying that he is a kinsman of the lord of Lothlorien after they ask him to be blindfolded as well
2. both Thranduil and Legolas were titled as princes of the Sindar in LOTR's Appendix.
3. Their names(oropher,Thranduil,Celeborn,....) are similar to each other


That's all I can remember
As for my own opinion I'm rather conflicted on this as Tolkien said nothing about them so I'm kinda hesitant to accept that they are but there's still so many hints that approve of the theory

Also I've always questioned this that if Oropher and Amdir aren't related to Thingol,Silvan elves that still somewhat think of Thingol as their overlord (considering that Denethor accepted Thingol as the high king of the Sindar), choose them as their kings and not Celeborn and Galadriel or even Elrond

(This is all purely my opinion and guesses and I'm not suggesting that they're canon)

 

[Olorgando]

1. Legolas saying that he is a kinsman of the lord of Lothlorien after they ask him to be blindfolded as well

Not quite. He simply say "I am an Elf and kinsman here," but does not claim kinship with anyone specific.

I can only find Oropher mentioned in J.E.A. Tyler's "The Complete Tolkien Companion", 3rd edition (possibly 2002), where he is called "One of the Sindarin princes who emigrated eastwards after the Fall of Morgoth to rule forest realms of their own." Also that he was Thranduil's father, and fell in the War of the Last Alliance at the end of the Second Age.
Robert Foster in his "The Complete Guide to Middle-earth" (2003 hardcover illustrated edition) has no entry for Oropher. My index volume for all twelve volumes of HoMe also has no entry. Same for the index to LotR at the end of RotK.
Oropher seems to be named for the first time in writings found in the "Unfinished Tales" of 1980. But Tyler above seems to have collected everything written there in the entry of his "Companion".