The probability is #2/6#, or #1/3# if you need to simplify.
Explanation:
Since a cube has #6# sides, and she wants to roll either a #1# or #2#, she has two favorable outcomes. Her chances are is #2# out of #6# outcomes (to roll either a #1# or #2#).
#2/6# outcomes is equal to a #1/3# chance of a favorable outcome.