The Jedi were almost extinct, only Yoda and Obi-Wan remained... As their last shot, they trained Luke to become a Jedi and defeat Vader and the Emperor.
Later, Yoda states that if he failed, there was another hope. Leia, the twin sister, could be used too. But, since this was their last chance, and Luke was veery weaker than the Sith,
Why didn't they train both Luke and Leia?
It would be a lot easier to beat Vader if they attacked together. Obi-Wan could have just asked, in the force-vision at the Hoth system, for Luke to bring Leia with him to Dagobah...