I don’t think that Edmund, Peter, and Lucy were upset about Susan not going to Aslan’s Country with them.
Hear me out. Aslan’s Country is supposed to be Lewis’s Narnian equivalent of Heaven. In the Bible, Heaven is described as being a place of eternal life and happiness where there is no misery or sorrow. Aslan says in the last book “You do not yet look so happy as I mean you to be” (can’t give the exact page number because I’m using my ebook but it’s on the last page of the book). Aslan’s Country is meant to be this land of eternal happiness, this eternal happily ever after. The Pevensies have an eternity to go on adventures and meet people from the past and learn new languages and do so many things and have all the time in the world to do so. So, they’d be a bit too distracted to worry about their sister.
But, I feel like the most likely reason as to why they wouldn’t be too upset about Susan not being with them is because they had faith in Susan. They believed that Susan never truly forgot about Narnia, she was just hurt about being sent back to England - and they understood that. They, too, had been hurt by being sent back to England when their only wish had been to remain in Narnia. They understood that Susan’s coping mechanism was to push all thoughts of Narnia out of her mind. As someone with anxiety, I, personally, find it much more helpful to push all negative thoughts out and act like my problems don’t exist instead of actually dealing with them. While not the healthiest route, it definitely is the easiest.
By “forgetting” Narnia, Susan doesn’t have to deal with those emotions of betrayal and hurt because, in her mind, there’s nothing to feel betrayed or hurt about. Peter, Edmund, and Lucy are all very perceptive people, I would be surprised if they weren’t aware of the truth behind her “forgetting” Narnia.
They all felt betrayed and hurt by Aslan for pushing them out, but Edmund, Lucy, and Peter all eventually realized that it’s a test of their faith. I don’t necessarily believe that Aslan himself was testing their faith in him but I do believe that being pushed out of Narnia multiple times was their test of faith. Peter, Edmund, and Lucy were just able to overcome those doubts in Narnia and Aslan and come out with an even stronger faith. Susan, someone who had had doubts since the beginning, only had her faith crumble.
But there’s this sense of anger to her that her siblings probably noticed. Lewis said himself that he had been angry at God for not existing which was his argument for turning to atheism. Susan is going through that same process. She’s angry, she’s hurt, and it’s worse because she’s so blinded by her anger and despair that she doesn’t see her siblings going through the same process as her which makes her feel worse.
In all of her claiming to “forget” Narnia, it’s pretty obvious that she remembers. Her siblings know that she remembers. And while the other Friends of Narnia may believe that Susan is a lost cause, Susan’s siblings don’t. Peter, Edmund, and Lucy are the only ones to not say anything about Susan and her disbelief in Narnia (except for Peter’s “She’s no longer a friend of Narnia” which he says in a more disappointed manner than anything). Her siblings are disappointed that Susan turned her back on Narnia, but they still have faith in her. They believe that she will eventually return to her belief in Narnia, only that time it will be stronger than ever.
Nobody understood Susan better than her own siblings. They knew that they couldn’t force her to believe in Narnia, she had to do it herself. She couldn’t be forced into believing in Narnia, she had to come willingly - and her siblings understood that, too. And they knew that her “forgetting” wouldn’t last very long. She would come around, she just had to do it in her own time.
So, no, I don’t believe that her siblings were ever very upset over Susan not coming to Narnia with them during their deaths. It was never a matter of “if” but rather a matter of when she would join them in Aslan’s Country.
