It might be Cassiopeia. In Quenya Wilwarin means Butterfly.
It has been suggested that the Seven Butterflies, the Seven Stars, perhaps was the Pleiades but I personally doubt it. It has also been mentioned that the Seven Stars became Valacirca but that is not the conclusion from Bolt1, Coming of the Elves. ( *1.) and it is not what can be concluded from the Sil. ( *2.) Beneath is why I trust Wilvarin to be Cassiopeia.
*1. " Yet the Seven Stars were not set by Varda, being indeed the sparks from Aulë`s forge whose brightness in the ancient heavens urged Varda to make their rivals " This also support the idear of them being Cassiopeia; that the butterfly were bright.
*2. In the Silmarillion Wilvarin and Valacirca are specifically mentioned one by one and can thus not be the same." and many other of the ancient stars she gathered together and set as signs in the heavens of Arda ; Wilwarin, Telumendil, Soronúmë, and Annarríma; and Menelmacar with his shining belt, that forebodes the Last Battle that shall be at the end of days" (...And high in the North as a challenge unto Melkor she set the crown of seven mighty stars to swing, the Valacirca the sickle of the Valar and sign of doom ) This also means that the stars in Wilwarin was already present in the sky but Varda set them together and they were thus not new stars.
In one of the poems, Kortirion among the Trees the story goes "And oft they dance beneath the roofless sky, When naked elms entwine in brancing lace The Seven Stars, and through the boughs the eye stares golden-beaming in the round Moons face" In my opinion the Seven Stars has to be high in the sky in order to be seen when you are standing among trees. And that is Cassiopeia and there is seven bright stars whereas the Pleiades are lower and harder to see, even harder when standing among trees. Furthermore Cassiopeia can be found nearly overhead in autumn, same time of year as the poem refers to and as a little curiosity the real bright stars in Cassiopeia forms the letter W. ( Which again could be seen as two wings like a butterfly ) The one thing I can see really contradicting it should be Cassiopeia is that she is made of five bright stars but there is two more smaller stars within her image making it seven in all.
The Pleiades, the seven sisters, is very close to each other from our point of view and it basically demands a clear moonless night and a keen eye to observe all seven. The star Electra in the Pleiades like to hide her face according to the "real" legend out of sorrow at witnessing the destruction of Ilium. And in the Poem is not a moonlees night, in fact the Moon is full and shines brightly.
Valian Years of the Trees.
V.Y.t.1000-1050. Wilwarin was set in the sky in Varda`s second making of stars. From the vats of Telperion she took silver dew and made new and brighter stars against the coming of the Firstborn.
Related links.: Battles : Prophecy of the Last Battle. Encyc.: Wilwarin., Stars, the prophecy and the creation.