In the 2nd teaser trailer for The Force Awakens, from 0:33 to 1:00 we hear Luke say:
The Force is strong in my family. My father has it. I have it. My sister has it. You have that power too.
The only person he could be referring to with "You have that power too" is Rey, so this strongly implies that she is also part of "my family" (Luke's family).
If that weren't enough, the shot at 0:53 also seems to show Maz Kanata handing Luke's light saber to Rey shortly before "You have that power too".
Se also this answer.
Given this, I'm surprised that there's still so much doubt about whether Rey is a Skywalker. What have I missed?
UPDATE: Yes, I know that that dialogue is a mix from the scene in Jedi where Luke tells Leia they are brother and sister, and that it doesn't appear in The Force Awakens. However, IMO that doesn't reduce its significance.