I can't see the picture....
But why aren't you using PARALLELPERIOD instead of your cousin>ancestor.lag(1) thing above? It's simpler....
[Code="Other"]
SUM(
PARALLELPERIOD
([Time].[Year - Quarter - Month - Date].[Year],
1 ,
[Time].[Year - Quarter - Month - Date].CurrentMember),
[Measures].[Sales Discount])
[/Code]
🙂