Find all ancestor nodes of a HierarchyId using a single SQL statement?
To get "all parent nodes of a given node":

select *, position.GetAncestor(1), position.GetAncestor(1).ToString()
from employee
where position=hierarchyid::Parse('/3/1/')

EmpId  EmpName       Position  (No column name)  (No column name)
5      VP Financing  0x7AC0    0x78              /3/


but there will only ever be one due to the nature of hierarchies.

If you really want to get all immediate children nodes of a given node:

select * 
from employee 
where position.IsDescendantOf(hierarchyid::Parse('/3/1/'))=1
      and position.GetLevel()=hierarchyid::Parse('/3/1/').GetLevel()+1

EmpId  EmpName              Position
6      Accounts Receivable  0x7AD6
10     Accounts Payable     0x7ADA


-- EDIT 

I see that you want all ancestor nodes. Perhaps try an approach like this:

select * 
from employee
where hierarchyid::Parse('/3/1/2/1/').IsDescendantOf(Position) = 1


or

select * from employee
where ( select position 
        from employee 
        where empname='Accountant 4' ).IsDescendantOf(Position) = 1


here is a CTE method for comparison:

with w as ( select * from employee where empname='Accountant 4'
            union all
            select e.*
            from employee e join w on(w.position.GetAncestor(1)=e.Position) )
select * from w;