4Game

It is related to Slacks' video DOTA LOREGASM: Planetfall: The Key to it All
, but I believe that he wrongfully pointed out that the sister earth is DOTA Allstars, because a sister earth does not mean an alternate version of the same earth but something similar but different, and it is more likely the DOTA Allstars is just an alternate timeline.

More to the point, the sister earth that was mentioned is Heroes of Newerth HoN, and voice lines of feeling regret to what happened to HoN is maybe, just maybe what Icefrog feels about HoN, like:

I cannot undo what has been done. I cannot save those who are lost. But I will learn how to cross such great distances, and avenge the wrongs done my sister earth!

There are others of his kind who share blame for the death of my sister earth, and perhaps countless other worlds bereaved of champions.

And with HoN's developers announced that the game will be shut down on June 20, 2022. We can expect / hope that Valve already made request some of HoN's heroes to be ported to DOTA 2 because Icefrog "allegedly" worked for S2 Games as a programmer on Heroes of Newerth, and maybe he want some of his creations back to him.

And from what I have seen from Valve's great work on the event games in last two years, I believe personally that the next update will be huge, and there will be rework to other heroes beside Techies like Meepo.