?In the desolate future, when an isolated and curious teenager is suddenly forced from his collapsing underground bunker into the overgrown wilderness outside, he will have to survive long enough to make a few allies if he is to find his little sister, rumored to be alive.?

Best to describe the events in order i.e inciting incident (sister is taken) then the action (fight for, search for or bargain with to get her back) goal (get the sister back).

For example:
After his sister is taken by a lawless gang in a post apocalypse desolate future, a [good description] teen must [compelling action] to get his sister back.

The teen's objective goal is to find his sister.??Okay. The need to survive is self-evident; it goes without having to say so.. ?Finding allies is a?means to that end.?

Why does he need allies??? I presume to fight whoever is holding captive his sister.? Well,? who is the antagonist?? From whom , specifically, must the teenager rescue his sister -- before it's too late?

I agree.