Why do Mormon missionaries travel in pairs? Why do Mormons travel in pairs (husband and wife, two young women, or two young men) instead of a group? Does a pair that consists of a brother and a sister ever go on a missionary trip together?
There are many reasons why we send our missionaries out "two by two". 
First of all it was given by way of revelation to Joseph Smith that he should send out missionaries in this matter.


  Doctrine &amp; Covenants 42:6 And ye shall go forth in the power of my
  Spirit, preaching my gospel, two by two, in my name, lifting up your
  voices as with the sound of a trump, declaring my word like unto
  angels of God.


This command was also given by the Saviour himself when he sent forth his apostles "two by two" proclaiming his gospel to the people. 


  Mark 6:7 And he called unto him the twelve, and began to send them
  forth by two and two; and gave them power over unclean spirits;


It is not always done two by two. If an odd number of missionaries are in a certain area, a group of 3 may be organized though not common. Fulltime missionaries will often invite regular members to come along with them on their visits. Fulltime missionaries are never permitted to evangelize by them selves. There are also many practical reasons for this. Safety being first and foremost.