1-a.
I borrowed two books from the library.
One
is a biography, and
the
other
is a nonfiction.
["The other" refers
to one "book," which is a singular noun]
1-b. I borrowed
three books from the library.
One
is a biography, and
the
other
books are nonfiction.
["The other" refers
to "books," which is a plural noun]


2-a. I have two sisters.
One
of my sisters is a doctor, and
the other
sister
is a teacher.
["The other" refers to "sister,"
which is a singular noun.]
2-b. I have
three sisters.
One
of my sisters is a doctor, and
the
other sisters
are teachers.
["The other" refers to "sisters,"
which is plural a noun.]
3-a. Of the
three books I borrowed from the library,
one
is a biography, and
the
other books
are nonfiction.
["The other" refers to "books,"
which is plural a noun]
3-b. Of
the three books I borrowed from the library,
two
are biographies, and
the
other book
is nonfiction.
["The other" refers to "book,"
which is a singular noun.]