[Mithalwen]

For me the most interesting thing about sisters in the books is their absence.
It occured to me that I could think of no portrayal of a sister relationship. Some of the male characters have sisters but none of the women do - that we see at least - other than the minor character of Ioreth, which is a rather onesided thing. The remarkable daughters of the Old Took are mentioned (which may have been a bibliographic reference to Tolkien's mother and her sisters whose father was remarkably longlived if I remember correctly) but not seen. All the major female characters either have brothers or are only children - the exception being Niniel/Nienor whose sister died before her birth.

And none of the women seem to have surrogate sisters and are often motherless - they are women in a world of men. Think of how isolated Arwen is in the original descriotion of her.

So while I do not see the significance in the same way as Son of Numenor, I do think it is an aspect of the overall portrayal of women and female roles - more significant in their absence than in their presence ...