The variable G.Edges.EndNodes of a graph can't be modified, it is always sorted by the nodes in the graph. This is so that the presentation is standardized, meaning that two graphs that have the same nodes and edges will present as the same.
You can save the sorted edges table into another variable, though:
weightSortedEdges = sortrows(g.Edges, 'Weight')
if this index is captured as the weight of each edge, or based on the index variable:
[~, ind] = sort(index);
indexSortedEdges = g.Edges(ind, :);