SSAS calculated measure with unary operator

Question

I have an account dimension with a parent-child hierarchy. I need to flip the signs on some of the leaf levels, so I created a calculated measure to handle this, and it works great except for one issue. The account hierarchy uses unary operators to do the proper addition/subtraction of siblings. When I use the calculated measure shown below, it no longer honors the unary operators. Is there a way to make the calculated measure honor those?

CREATE MEMBER CURRENTCUBE.[Measures].[Flipped Activity]
AS 
IIf(IsLeaf([Account].[Accounts].currentmember),
   ([Measures].[Activity] * [Measures].[AH Flip Value]),
   SUM([Account].[Accounts].currentmember.children, [Measures].[Flipped  Activity])
), 
FORMAT_STRING="$#,##0.00;($#,##0.00)",
VISIBLE = 1;