NAH when I break things down.

Just trying to get my understanding

Annie was 6 or younger when her mother died based on it being 3 years between Mother's death and husband's remarriage.

So Annie was 9 when Amy was born. And at this point Amy is just as much his daughter as Annie.

Amy grows attached to a necklace that her Dad wants her to have. A necklace that is a memory of his first wife lost to him and gifts it to his daughter because she showed it value and he gets to remember his past wife while also honoring his daughter.

At this point Annie would be 14. Something tells me she was not ever attached to this necklace at the time but was going through her own struggles as a teenagers and feeling disconnected. If Annie had a high regard for the necklace at that time, I do not see him gifting it to Amy.

I do not think he ever thought it would be such a point of contention based on your story, however when looking at the ages of the event, I would say grace needs to be given to all parties.

I would guess that Annie works through her anger and finds peace with her father. I feel it has much less to do with the necklace and more to do with seeing her father move on and a variety of other emotion baggage she and he likely dealt with leading up to her mother's/his late wife's death.