Unity function for sorting objects by their position in the hierarchy.

OrderByPositionInHierarchy.cs

public static IEnumerable<T> OrderByPositionInHierarchy<T>(this IEnumerable<T> enumerable) where T : Component
{
    return enumerable.OrderBy(comp => new HierarchySortable(comp.gameObject));
}

/// <summary>
/// Sorts a list of gameobjects by their position in the hierarchy (top to bottom as if the entire hierarchy is expanded)
/// Example: sortedGameobjects = unsortedGameobjects.OrderBy(go => new HierarchySortable(go)).ToList();
/// </summary>
public struct HierarchySortable : IComparable<HierarchySortable>
{
    private List<int> _parentSiblingIndexes;

    public HierarchySortable(GameObject gameObject) : this()
    {
        _parentSiblingIndexes = new List<int>();

        if (gameObject.transform.parent == null)
        {
            _parentSiblingIndexes.Add(gameObject.transform.GetSiblingIndex());
        }
        else
        {
            Transform parent = gameObject.transform;

            while (parent)
            {
                _parentSiblingIndexes.Add(parent.GetSiblingIndex());

                parent = parent.parent;
            }
        }

        _parentSiblingIndexes.Reverse();
    }

    public int CompareTo(HierarchySortable other)
    {
        int aCount = this._parentSiblingIndexes.Count;
        int bCount = other._parentSiblingIndexes.Count;

        for (int i = 0; i < Math.Max(bCount, aCount); i++)
        {
            bool aMaxed = i >= aCount;
            bool bMaxed = i >= bCount;

            if (aMaxed && !bMaxed)
                return -1;
            else if (!aMaxed && bMaxed)
                return 1;

            int a = this._parentSiblingIndexes[i];
            int b = other._parentSiblingIndexes[i];

            if (a > b)
                return 1;
            else if (a < b)
                return -1;
        }

        return 0;
    }
}