When the line "that boy is our last hope" is said, Yoda gets bathed in a greenish blue light, representing Luke. However, when Yoda says, "no, there is another," he gets bathed in a red light, representing Darth Vader.
You must remember that every element in a movie is an intentional choice, and that this isn't the only time that these colors are used to represent their affiliated characters, and let you know who is being discussed. The point here being that Darth Vader is the other. This makes sense since Darth Vader is actually the Hero of the story, in other words, it's about his journey and his path towards redemption. The story is merely being told through Luke's eyes.
This is why it is Darth Vader who finds his place among the Jedi Master by the end of the story. Furthermore, the thing people use to justify Leia as being the other hope is just Obi-Wan's quote, but as you'll recall, he didn't know who Yoda was referring to any more than the viewer did, and this is his own misconception, one which is used as a misdirect for the viewers. Luke is not present for all key elements of the story, which involve the struggle in the balance of the force, only Darth Vader is, because it's his story.
There is also Yoda’s last words, which echo his line in Empire, and which he says after Luke has learned that Vader is actually Anakin Skywalker:
YODA ... There is...another...Sky...Sky...walker.
The fact he says this after Vader acknowledges he is a Skywalker and not before, should also let you know he wasn't talking about Leia.
OBI-WAN: I will take the child and watch over him. Master Yoda, do you think Anakin's twins will be able to defeat Darth Sidious?
YODA: Strong the Force runs, in the Skywalker line. Hope, we can . . . Done, it is. Until the time is right, disappear we will.
This line doesn't mention Leia as an additional hope, it mentions BOTH TWINS as the original hope. Further proof that it is Darth Vader who is the other hope.
Furthermore, Obi-wan spoke with certainty all the time in the prequels, and was constantly wrong.
Look at the prophecy of the chosen one: "Another depiction of the prophecy was a cave painting in the Ophuchi Sanctuary. According to the Ophuchi interpretation, the painting showed an enslaved dove rising up and defeating a serpent that slowly tried to corrupt and destroy him to catch him as its prey. When the serpent was destroyed, rain that had not fallen in the desert for many years would fall again. The Ophuchi believed that a drought would overtake Tatooine the day that the Chosen One was born, and that he would be aided in his task by one called the Argus who would free the Chosen One from his shackles before vanquishing the serpent."
The enslaved dove, is Darth Vader. The serpent trying to corrupt him is Emperor Palpatine. The Argus is Luke, who causes Darth Vader to rebel against Emperor Palpatine. So you are telling me, YOU honestly think, the PERSON DESTINED TO RETURN THE BALANCE TO THE FORCE, who GEORGE LUCAS CONFIRMED WAS "officially the Chosen One and not Luke," wasn't the other hope, but Leia was?