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.
Leia wasn't the other hope.
Got to say, even when ben explains that he THINKS its Leia, he, himself, is unconvinced. He is like, you were our last hope. The other hope? He probably meant your sister and she's poop, so we are all doomed. Thanks Luke, way to blow it.