With this apocalypse between them

How do I tell them that one day they may not speak?

That no matter how close as sisters and friends they may be with one another growing up, one day one of them may leave, to return but to never live here again, and how this will start an irreversible chain of events. Events that will propel the one who stayed into the cold vacuum of space where no one else can see; where no one else can know. Where black ice and rock will collect and grow...

Years will pass while the Mama and I love each of them proudly, marrying their hopes and realities to our own pasts, their futures, until one day the one who stayed will hit a wall at full speed, while the one who left will try to tear it down. Misunderstandings, mistrust and betrayals will then cause a rift in the entire family; the rubble of the wall too heavy to clear.

And at the same time, somewhere deep in space, all that black ice and rock will become something behemoth that one day falls back to earth. It will reenter the atmosphere close to the speed of light, under the cover of night, and lay waste in its wake, boiling oceans and cratering continents. Fires will burn uncontrollably for seemingly a millennia. The sky will be a perpetual ceiling of gray ash.

With this apocalypse between them, one day something may happen to the Mama and me, we may get sick, and there will have to be a coming together, a temporary reckoning in order to take care of family business.

We will tell them both, "She's your only sister."

And one will say, "That's not the way it works."

And the other will say, "I never did anything wrong."

And we will both say, "But she's your only sister, and we love you both."

And then the first one will say, "But blood does not trump the healing reciprocity of forgiveness. I can only do for me, not for both."

And I will think, you're right, it does not, and you can only do for you, as your sister can only do for her. Nor does any of this require a Hollywood ending where uplifting music will play and hugs and tears abound. But I know we will still love them both no matter what. 

In the shadowy distance, a green shoot will spring from a dry, blackened crater.

So how do I tell them all this some day?

And do I?