When she visits the new home, is there a room

just for her, with all her old stuff?  Can she still feel she has a place in this family? How delighted is the sister when they are together?

Since you moved after she went to college, all the places and things associated with her childhood are no longer there.  Her attachment to these memories is more pronounced because they occupy a larger portion than the love received from her parents.  These holes can only be filled at certain age.  If the parents were not there, the child would fill the holes herself, with things that are mostly bad, sadness can be one of them.

She is an adult now.  But most of the things that go through our mind are not thoughts but reflexes, conditioned in childhood.  The circumstances are long gone, but the reflexes stayed, controlling our reactions to the present stimuli.