where children in heaven get to pick their parents and a sister is separated from her brother because of this. a literal "stairway to heaven" exists within, and while i have not googled this title, wondering what it actually is? and i believe this is not the title...

The movie is For Heaven's Sake (1950).

yes, i believe you are correct. while i did get some of the specifics wrong, i may have, at the time, just watched an Abbot and Costello with the stairway to heaven visual. thanks!