I wonder if it has to do with some factor unknown to us now that has to do with receptivity. Or possibly intention. Maybe one person is more receptive than another. Maybe it has to do with conditions in the atmosphere. Maybe it has to do with having a message to convey. Here are some examples.
In the death of my friend Gini a few Septembers ago, I heard from her immediately and so did her son. In both cases the contact was imaginative, creative, beautiful and artistic in its expression. We marveled at how clever and witty it was--but of course--so like her. She was an artist.
Yet her daughter, to whom she was very close, has not heard from her. It is just the oddest thing. I know that if Gini could contact her she would. So it makes me wonder if it has something to do with receptivity.
In the death of my neighbor's dog Crumpet, I heard from Crumpet a few days after she crossed over, but my neighbor herself did not. She and Crumpet were very close. Her husband had some dreams where she came to him a few days later. In the case of contact with me, however, Crumpet had something specific to tell me. When I was eventually able to tell the story to my neighbor (it was so emotional for me I could barely articulate it, even weeks later), my neighbor confirmed that Crumpet had been trying to come to my house for weeks before she died. She was so old and blind, though, that she was only able to make it part of the way each time. It means the world to me that she tried so hard. The message that she conveyed was very meaningful because I was truly unaware.
When my brother's cat crossed over, he visited me in a dream a few nights later to show me his new "digs." He gave me the grand tour and I saw the house he was living in from a cat's point of view, 10" off the floor. The dream was very long and I spent a good deal of time with him. I really got a feel for his new "human." I even saw her walking across the house from the cat's point of view (her heels!). I experienced how happy and comfortable he was in his new home.
So the kitty contacted me but not my brother. Why? My brother would have loved that reassurance yet it was I who was visited and not him. I told my brother the story but at the time I do not think he was very receptive to it. Over the years he has changed, however.
Now, putting the shoe on the other foot, my brother had contact with my beloved Mitzi after she crossed over. He actually saw her. She materialized in front of him and played with him briefly. I was in the same room and saw him interacting with her but I did not see her materialize. In fact I did not even know what happened until he told me.
I had many other afterlife experiences with Mitzi but none that involved materializing except for when she appeared in a photo with Bitsey, my other black cat who crossed over.
So from these examples, you might suggest to your sister that it could possibly have something to do with the conditions for contact. It also might have something to do with intention, i.e., something specific to communicate.
Cher
p.s. I enjoyed reading everyone's stories.