Posted by Peter d'Owdon on October 28, 2009 at 18:47:46 user Dowden.
The books are not explicit as to which of Molly or Jim is younger, but I think I have uncovered a clue (which might already be well known): The Billies refer to the pair as "Miss Turner she was then..." [and] ..."Master Jim" (SA 13). In the convention of the times, oldest children would retain the title 'Miss or Master [surname]' while subsequent children were called 'Miss or Master [first name]' -- this implies that Molly is the older.
In the Japanese translations Jim is described as being the younger brother to Molly Turner. An interesting feature of Japanese is that there are separate words for kinship terms based on relative age, like older brother, younger sister etc, rather than just "brother". This means any translator has to determine relative age before translating these words in to Japanese.