NO! The index panel at the family tab does NOT do a secondary sort on birthdate! When selecting the primary sort option "Family, Given Name", the individuals are sorted first by surname, then by given name, but the birthdates are not sorted in ascending or descending order. I suspect they are based on the individuals' reference ID number, which is not helpful.
I am not referring to the F2 pop-up index of individuals.
Are you speaking about FTM 2010 or something else? I am using Family Tree Maker 2010 for Mac, ver. 18.104.22.168