1- Do you start introducing "other" and "another" as pronouns or determiners? Perhaps presenting them first as determiners helps the students understand them as pronouns more easily.
2- I too think that "the other" has a boundry, ofcourse not the boundry of including only two items in a group.
3- though "another" has no boundry it moves in a slow pace! Each time it can refer to only one item.
Student or Learner