reftex-index-phrases-sort-prefers-entry

reftex-index-phrases-sort-prefers-entry is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Non-nil means when sorting phrase lines, the explicit index entry is used.
Phrase lines in the phrases buffer contain a search phrase, and
sorting is normally based on these. Some phrase lines also have
an explicit index argument specified. When this variable is non-nil,
the index argument will be used for sorting.

You can customize this variable.