Unfortunately, yEd cannot do that. Moreover, I don't think such a feature can be easily implemented. For instance, what does sorting alphabetically mean in 2D? And how would you do that without changing the enclosing group? For any sorting scheme you propose, it should be easily possible to construct an example that would result in the enclosing group being changed.
(Keeping the position of groups is usually not enough, because size changes may easily lead to bad overlaps.)