No, this is not possible. There is currently no way to check what object is active in a container. To get around this problem, I usually just create buttons and hide/show groups of objects based on those buttons--i.e. I just avoid the container altogether. Depending on client requirements, I sometimes make the buttons look like container buttons so users may not even know the difference.
I think the only way to do this is with a macro.
Set obj = ActiveDocument.GetSheetObject("CT01")
Set p =obj.GetProperties
ActiveDocument.Variables("vIndex").SetContent vIndex, true
And set the conditional display expression of the text box to something like if(vIndex=2,1,0) to show it when the third object in the container is active and otherwise hide it.