Not sure if you still need an answer to this, but here's a shot at it:

With Hierarchy (col1, col2) AS

(

SELECT 1, NULL

UNION ALL SELECT 2, NULL

UNION ALL SELECT 3, '4'

UNION ALL SELECT 4, NULL

UNION ALL SELECT 6, NULL

UNION ALL SELECT 8,'9 ,10 ,11 ,12 ,13'

UNION ALL SELECT 9, '10 ,11 ,12 ,13'

UNION ALL SELECT 10,'11 ,12 ,13'

UNION ALL SELECT 11,'12 ,13'

UNION ALL SELECT 12,'13'

UNION ALL SELECT 13, NULL

UNION ALL SELECT 18,'19 ,20 ,21 ,46 ,47'

UNION ALL SELECT 19,'20 ,21'

UNION ALL SELECT 20, '21'

UNION ALL SELECT 21, NULL

UNION ALL SELECT 23, NULL

UNION ALL SELECT 26, NULL

UNION ALL SELECT 27, NULL

UNION ALL SELECT 28, NULL

UNION ALL SELECT 29, NULL

UNION ALL SELECT 30, NULL

UNION ALL SELECT 34,'35 ,36 ,37 ,38'

UNION ALL SELECT 35,'36 ,37 ,38'

UNION ALL SELECT 36,'37'

),

AdjacencyList (col1, col2) AS

(

SELECT col1, col2=Item

FROM Hierarchy a

CROSS APPLY dbo.DelimitedSplit8K(col2,',') b

),

Invoices (Invoice, HCC) AS

(

SELECT 1, 34

UNION ALL SELECT 1, 35

UNION ALL SELECT 1, 36

UNION ALL SELECT 1, 37

UNION ALL SELECT 1, 38

UNION ALL SELECT 2, 12

UNION ALL SELECT 2, 13

UNION ALL SELECT 3, 26

UNION ALL SELECT 3, 27

)

SELECT Invoice, HCC=MIN(COALESCE(b.col1, c.col2, a.HCC))

FROM Invoices a

LEFT JOIN AdjacencyList b ON b.col1 = a.HCC

LEFT JOIN AdjacencyList c ON c.col2 = a.HCC

GROUP BY Invoice;