The answer comes from the phrase "...and it shall be on the day that he bequeaths to his sons..." This phrase is superfluous since the law it states - that the firstborn gets a double portion - is expressed in different words in the very next verse. Therefore, the superfluous phrase teaches that the other brothers - if there are more than one - get at least as much as the firstborn, that is, at least 1/2, which is more than 1/3. Now that the 2/3 possibility is out, the only remaining logical alternative is that the firstborn gets double the portion of each brother.
Art: The Courtenay Brothers by Thomas Hudson