This may be too round-about of a way to get there, but maybe it can give some ideas.
I was having a hard time working with Groups, so I switched them to Sets.
Then I made a RegionCalc to get the Region for each state:
IF [Midwest] THEN "Midwest"
ELSEIF [Northeast] THEN "Northeast"
ELSEIF [South] THEN "South"
ELSEIF [West] THEN "West"
Then found the RegionCalc for the selected state:
IF [State]=[State Select]
Finally, made a conditional for each state if their Region matches the SelectedRegion:
Regional Filter Question2.twbx 423.3 KB