Chris was prepared to fight the invaders, at least well enough within the small time frame and limited resources he had access to. There is nothing more he can do before he loses the chance of saving the core.
Once enough preparations were done, he was on his way to Cromwell City.
Since he couldn't bring his sister with him, he placed her with his neighbors, the three girls who moved into the neighborhood less than a year ago. He spoke to them once in a while but it was his sister who had a good relationship with them.
They agreed to take care of her for a few days until he returned.
With Chris busy playing the game most of the time, Abby became friends with the girls. Unlike the aunties in the neighborhood, she was able to express herself freely with those girls as they were only a few years older than her.
So she started to spend her free time with the girls and became friends with them. Those three too treated her as their little sister.