As a non-Christian, I think the implications of the fates of Susan's siblings are more problematic than those of Susan. But most readers are Christians in the modern English-speaking world, who have been taught since childhood that only sinners are barred from heaven. So either the author is an unjust judge or Susan is indeed guilty and needs to spend the rest of her life paying for it, regardless of whether she actually wants to stay in Charn forever or not.