Animus, so your Tirian is now being played by Keanu Reeves
The fact is Lewis used Eustace, Jill, and Polly to discuss Susan. It may be so that people other than her siblings can comment about her. We will never know why he chose those three specifically.
However, we do know Eustace has known Susan for allof his life and Eustace has tried to talk with her about Narnia. We, in fact, are not ever told when they started to have these conversations or whether Susan ever talked about Narnia with Eustace or Jill or Polly. She more than likely did with her siblings and probably also with the Professor.
Eustace says: "and whenever you try to get her to come and talk about Narnia, she say, 'What wonderful memories you have! Fancy your still thinking about those funny games we ued to play when we were children."
Jill says: "Shes interested in nothing nowadays except nylons and lipstick and invitations. She always was a jolly sight too keen on being grown up."
Polly says: "She wasted all her school time wanting to be the age she is now."
The way they are talking makes me think they have a pat with her. Let's pretend the seven year gap exists between TSC and TLB, they would then have all that time to meet and greet, maybe share tales before Susan goes on her downward spiral. The less time you put beteen TSC and TLB, the harder the less they may have met, but it isn't impossible for them to have met for Winter break and Spring Break in the year after Jill joins the Friends group.
I am one for the obvious examples thaty were given are the ones that are correct. Why complicate the story that Lewis wrote just because of the perceived biases of the characters? No one contradicted them.
MrBob