Possible to derive the start month of prior sibling of selected Scenario in RTP?

Summary:

Hi there,

We have our regular scenarios and we have created Staged Scenarios for each of them.
These Staged scenarios deal with the periods prior to the open periods of the regular scenarios, and calculations within them are to be similar to the regular ones.
In our calculation, we require the closing balance of the ending period of the staged scenario to get populated to the opening balance of the regular scenario.

Is there any way to derive the start/endmonth for the prior sibling of scenario that the user select in run time prompt on the fix statement? I am aware that the planning formula getStartMonth[] does not work with any Essbase function, so I am wondering if there are other options we can use in Essbase script.