This will probably be unpopular, but here goes.
I think that the writers had grown weary of the girls characters and decided to take the easy way out and kill them off before they got to Terminus. Either what is going to happen at Terminus is so socially unacceptable that they could not include them in the story, or it was decided that they were the cast members most likely not to be missed so they got the ax.
My tipoff was when the two adults travelling with them started going off on walks, leaving them by themselves. Every time they would come back, the girls were under attack by walkers. So, what did the adults do? Go on longer and more frequent walks. I got the hint.
In the comics, there were two brothers (Ben and Billy) who had a somewhat similar storyline.
well, not everyone who scattered from the prison was going to make it. it seems more logical that young kids may be at more of a disadvantage. then one is crazy/in denial and goes off the deep end dangerously. no real way to fix that in this world.