Did Martin Luther and Zwingli not consider each other brothers in Christ? Did Martin Luther and Zwingli not consider each other brothers in Christ and members of the Body of Christ? Did they ever state that in a written form?
Did Martin Luther and Zwingli not consider each other brothers in Christ?

Definitely not.  Zwingli did offer a hand of brotherhood to Luther, but Luther refused it, telling Zwingli and the Swiss church "We cannot acknowledge you as brethren".

Roderick C. Meredith's history of the Reformation was recently serialized in Tomorrow's World magazine.  One section describes the final meeting between Luther and Zwingli, Martin Luther Unleashed:



THE FINAL MEETING OF LUTHER AND ZWINGLI

Schaff describes this meeting:


  On Monday morning he arranged another private conference between the
  Saxon and the Swiss Reformers. They met for the last time on earth.
  With tears in his eyes, Zwingli approached Luther, and held out the
  hand of brotherhood: but Luther declined it, saying again, “Yours is a
  different spirit from ours.” Zwingli thought that differences in
  non-essentials, with unity in essentials, did not forbid Christian
  brotherhood. “Let us,” he said, “confess our union in all things in
  which we agree; and, as for the rest, let us remember that we are
  brethren. There will never be peace in the churches if we cannot bear
  differences on secondary points.” Luther deemed the corporal presence
  a fundamental article, and construed Zwingli’s liberality into
  indifference to truth. “I am astonished,” he said, “that you wish to
  consider me as your brother. It shows clearly that you do not attach
  much importance to your doctrine.” Melanchthon looked upon the request
  of the Swiss as a strange inconsistency. Turning to the Swiss, the
  Wittenbergers said, “You do not belong to the communion of the
  Christian Church. We cannot acknowledge you as brethren.” They were
  willing, however, to include them in that universal charity which we
  owe to our enemies (Schaff, Philip. History of the Christian Church,
  Vol. VII. Hendrickson, 1996 (1888). pp. 644–645).


Thus we see that Luther parted from Zwingli, not in the feeling that the Swiss party was guided by the Holy Spirit, but that Zwingli was guided by a different “spirit” than himself. Indeed, there is ample testimony, even among Protestant writers, that the reformers did not have the “unity of the Spirit” that only God’s Spirit can bring.

Notice Plummer’s account of Zwingli’s desire to avoid this pathetic disagreement:


  But, there is no need to doubt his declaration that he had carefully
  avoided corresponding with Luther, because he says, “I desired to show
  to all men the uniformity of the Spirit of God, as manifested in the
  fact that we, who are so far apart, are in unison one with the other,
  yet without collusion.” They did not remain in unison, as all the
  world knows; and it is one of the many sad facts in the history of the
  Reformation that Luther declared Zwingli’s violent death to be a
  judgment on him for his eucharistic doctrine (Plummer, pp. 141–142).