Here is a code snippet that will initially sort an unsorted column, or reverse the sort if the column is already sorted.
SortColumnDescriptor scd = this.gridGroupingControl1.TableDescriptor.SortedColumns["ParentDec"];
if(scd == null)
{
this.gridGroupingControl1.TableDescriptor.SortedColumns.Add("ParentDec");
}
else if(scd.SortDirection == ListSortDirection.Ascending)
scd.SortDirection = ListSortDirection.Descending;
else
scd.SortDirection = ListSortDirection.Ascending;