I know it's originally from ROTJ, but think about it:
1. It doesn't make any sense to have that line in the trailer, unless the goal was to mislead the audience into thinking the movie is about Leia. While possible, it's highly unlikely because it wouldn't have made sense to present an old Leia as the hero. Besides, in that very same trailer, we don't see Leia. We see all the new heroes, including Rey. If Leia had a really significant part in the movie and was the point of the trailer, we would have seen her by then. For reference, said teaser:
https://www.youtube.com/watch?v=ngElkyQ6Rhs
2. The original quote in ROTJ is "You have that power too. In time, you'll learn to use it as I have. The Force is strong in my family: my father has it, I have it, my sister has it...
and... my sister has it. It's you, Leia." (here:
https://youtu.be/MDYX_PgorRY?t=93), while in the TFA teaser, it's: "The Force is strong in my family: my father has it, I have it, my sister has it... You have that power too." This is almost the same, yet not quite. Let's see why:
- Unless my ears are failing me, the line was actually rerecorded by an older Mark Hamill/Luke for the teaser (feel free to correct me if I'm wrong). This is an old Luke saying this to someone who can't be Leia. She's not dumb, she heard him the first time... 30 years ago!
- That cute little "and" in bold is a big sign. It exists in ROTJ because it signals the end of an enumeration. Luke gets his point across with that short, 3-item enumeration. He doesn't even really need to add "It's you, Leia." Removing it makes the enumeration feel incomplete in the teaser.
- Look at the order. In ROTJ, "You have that power too" comes first. In the TFA teaser, it's shifted around to become the conclusion of the enumeration as if Luke was saying: "My father, me, my sister, you (too)". It becomes the 4th item in the enumeration. True, the enumeration still feels incomplete without an "and", but that's what the "too" is here for instead.
I could still be wrong - in fact, I'd be happy to be wrong, and I still think it could be a red herring -, but that teaser has always bothered me because of that. Even knowing the original line, the way it's used in the teaser is engineered to make us think "Ooooh, I wonder who that "you" is!"