3. Allen:

"Celaeno, or Celeno, has been called the Lost Pleiad, which Theon the Younger said was struck by lightning!

It gives but one half the light of Taygete; still it has been seen with the naked eye, if a good one, and is so given in the Heis 'Verzeichniss'.

The Sister Stars that once were seven // Mourn for their missing mate in Heaven. (Alfred Austin)"

The number of the 'sisters' - if six or seven - surely cannot be determined by counting, there is no easy way to define where to stop, how to define the limit of visual magnitude below which the stars of the cluster should not be counted.

Instead it is a matter of conception, 6 there should be if the arrangement is idealized as hexagonal (with the 'hen' in the center):

7 they should be if the arrangment is thought of as the eggs in a Kingfisher's nest:

If they once were 7, then the 'sisters' should be formed like eggs.

A pair of missing stars are in some way connected with Ursa Major:

... Now the deluge was caused by the male waters from the sky meeting the female waters which issued forth from the ground. The holes in the sky by which the upper waters escaped were made by God when he removed stars out of the constellation of the Pleiades; and in order to stop this torrent of rain, God had afterwards to bung up the two holes with a couple of stars borrowed from the constellation of the Bear. That is why the Bear runs after the Pleiades to this day; she wants her children back, but she will never get them till after the Last Day ...

One reason for this is the form of the constellations:

"As the group outline is not unlike that of the Dipper in Ursa Major, many think that they much more deserve the name Little Dipper than do the seven stars in Ursa Minor; indeed the name is not uncommon for them. And even in our 6th century, with Hesychios, they were Σάτιλλα, a Chariot, or Wagon, another well-known figure for Ursa Major." (Allen)

South of the equator, where the constellations high up around the north pole are below the horizon, the 7 stars of Ursa Major should have less influence. Not only the Polynesians but also some peoples in South America counted the Pleiades as 6:

"The Abipones of the Paraguay River country consider them their great Spirit Groaperikie, or Grandfather; and

in the month of May, on the reappearance of the constellation, they welcome their Grandfather back with joyful shouts, as if he had recovered from sickness, with the hymn, 'What thanks do we owe thee! And art thou returned at last? Ah! Thou hast happily recovered!' and then proceed with their festivities in honor of the Pleiades' reappearance.

Among other South American tribes they were Cajupal, the Six Stars." (Allen)

We are reminded of the similar joy by the Polynesian women when Moon returned:

... When the new moon appeared women assembled and bewailed those who had died since the last one, uttering the following lament: 'Alas! O moon! Thou has returned to life, but our departed beloved ones have not. Thou has bathed in the waiora a Tane, and had thy life renewed, but there is no fount to restore life to our departed ones. Alas ...