documentclass[12pt,crop=true,border=1cm]{standalone}
usepackage[edges]{forest}
begin{document}

begin{forest}
  for tree={
    grow'=0,
    draw,
    forked edges,
    text width=10mm,
    minimum height=5mm,
    parent anchor=east,
    child anchor=west,
    text centered,
  }
    [A,tier=level1
      [B1,tier=level2,name=B1
%        [C1 would be here, but it is missing
          [D1,tier=level4,name=D1
            [E1,tier=level5]
            [E2,tier=level5]
          ]
          [D2,tier=level4,name=D2
            [E3,tier=level5]
            [E4,tier=level5]
          ]
%        ]
      ]
      [B2,tier=level2
        [C2,tier=level3
          [D3,tier=level4
            [E5,tier=level5]
            [E6,tier=level5]
          ]
          [D4,tier=level4
            [E7,tier=level5]
            [E8,tier=level5]
          ]
        ]
      ]
    ]
    % Draw the edges with the fork at the desired level.
    % The fork sep key specifies the "The l-distance between the parent anchor and the fork."
    % See Forest manual version  July 14, 2017, p81
    draw[red,thick] (B1.parent anchor) -- +(57pt,0) |- (D1.child anchor);
    draw[red,thick] (B1.parent anchor) -- +(57pt,0) |- (D2.child anchor);
end{forest}


end{document}

documentclass[12pt,crop=true,border=1cm]{standalone}
usepackage[edges]{forest}
begin{document}

begin{forest}
  for tree={
    grow'=0,
    draw,
    forked edges,
    text width=10mm,
    minimum height=5mm,
    parent anchor=east,
    child anchor=west,
    text centered,
  }
    [A
      [B1,name=B1
        [C1, phantom % C1 would be here, but it is missing
          [D1,name=D1
            [E1]
            [E2]
          ]
          [D2,tier=level4,name=D2
            [E3]
            [E4]
          ]
        ]
      ]
      [B2
        [C2
          [D3
            [E5]
            [E6]
          ]
          [D4
            [E7]
            [E8]
          ]
        ]
      ]
    ]
    % Draw the edges with the fork at the desired level.
    % The fork sep key specifies the "The l-distance between the parent anchor and the fork."
    % See Forest manual version  July 14, 2017, p81
    draw[red,thick] (B1.parent anchor) -- +(57pt,0) |- (D1) % you can remove option "red, thick"
                     (B1.parent anchor)    +(57pt,0) |- (D2);
end{forest}

end{document}