We know that:

1-Fang and Vanille were on crystal form which means they are very old but they appearance didn´t change.
2-Many humans from Pulse went to Cocoon after it was created.

It is therefore possible that Fang and Vanille´s husbands decided to depart against their will and even get divorced.Alongside with their child.

Vanille was not much young when she turned to crystal either.
Her age probably was enough to marry on Gran Pulse and have children.

They may not remember that either.


Judging from the character interactions and personalities I would say
that Fang is Claire and Serah´s grandmother.And Vanille is Nora´s mother.

Whether they are grandmother or grandgrand mother depend on many factors.