InitSiblings.cs

public void InitSiblings(List<WallGeometry> possibleSiblings)
{
    var filtered = possibleSiblings.Where(wall => wall != this);
    Siblings = filtered.Aggregate(new List<WallSibling>(), (walls, wall) =>
    {
        var point = SU.RealIntersection(wall.Basis, Basis);
        return point == null ? walls : walls.Concat(new List<WallSibling> { new WallSibling(point.Value, wall) }).ToList();
    });
}