Don't underestimate your boss and what is going on behind the scenes that you don't know, meaning that your co-workers' performance and attendance are being noted by your boss.

Keep doing the good job you are doing, without complaint and you will emerge the victor.
Employees with poor behavior, poor performance, and excessive absenteeism are orchestrating their own exit, even if you see no evidence of that.

When you are overwhelmed with work for any reason, it is okay to let your boss know that, without placing blame, just acknowledging how much you have to do. Managers need to know this because it is their job to manage the work and meet the deadlines.

All that said, I can understand that you are in a difficult spot between worker A and B, but you are not a chump -- you are a valuable employee!