I believe the siblings' new ability is [Rewind], as it has been hinted at multiple times already.

In the preview livestream, Vivian posed a thought-provoking question: "If you could go back, what would you be willing to give up?"

As early as the prologue, subtle hints suggested that the siblings possess the ability of [Reload]. Even in the trailers released prior to version 1.4, [Rewind] was mentioned, yet we know there was no actual [Reload plot] in version 1.4.

Now, with version 1.6 set to be the first half of the finale, it’s possible the siblings will unlock the [Rewind] ability. In the second half, we might rewind to a pivotal moment from earlier in the story.

When everyone expects the narrative to move forward, a sudden rewind would perfectly align with Hoyoverse’s penchant for defying expectations. Who knows? We might even encounter one of the Void Hunters along the way!