MDX Allow member set only siblings in Roles-Dimension Data

Question

Is it possbile to restrict a Role in Dimension Data by allowing him to see only siblings in the parent-child Hierarchy? Example Hierarchy:

EMEA              100

  UK               50
     London        30
     Southampton   20

  France           50
      Paris        10
      Lyon         40

To see only:

  EMEA             100
     UK             50
     France         50

Answer 1

I've received the expected result with the following MDX statement

 NONEMPTY(
    descendants(
       [Dim Branch Hierarchies].[Branch Hierarchy]
       ,,leaves
    )
  , (
     [Measures].[GrantedHi]
    ,StrtoMember( 
        "([Dim Users].[Account Name].[Account Name].["+ Username()+ "])" 
     )
    ) 
)