Which book is this?

My nan bought me a book back in the 80's. It was the last of a trilogy and I never read the first two. The story was told by a princess and the world on which she lived royalty had auburn hair and the working class were blonde. The queen through special bracelets was able to harness the power of the sun but there was also something genetically different about her to do so. She had to through a life changing event in order "to change". The story starts off with the princess waiting at the foot of a mountain for her sister who is overdue. The sister had gone off on a hunt in order to change and it is implied that she has died like another sister. The princess meets a hunter instead, shares a meal with him and then heads back to the palace. There she finds her father is leaving because he can't bear seeing his daughters die. There are some other humanoid aliens living on this planet and are more scientifically advanced and are able to test the daughter and find she is missing this genetic component. The queen then takes the hunter for a mate and becomes pregnant. The princess, hunter and scientific alien then go off on a quest to find her paternal grandfather who was kidnapped by aliens years ago and cloned. My memory gets hazy now but there's something about cat like humanoid aliens called something like silth/ seeth and the grandfathers consciousness has been woven into silk by spinners. Anyway she returns home with the silk accepting that she won't be queen to find her mother has given birth to a daughter who will be queen.

If it helps I got the impression that the first two books are about the father and mother. The alien race who abducted the grandfather would clone their captives and then place the babies on different worlds and then harvest them when they are older. The mother changed by fighting off the aliens when they came back for the father.

Any ideas?