Traditionally, male dogs are the alpha in a multi dog household. When you got your younger dog, the older male was automatically alpha since he was male and an adult. Now that he is not in good health, she feels she should be the alpha since she is healthy and young.
However, the older male will not voluntarily give up the boss position, so fights occur. If the male was to submit to the female, then the fights would stop. So to answer your actual question, dogs will attack older, sickly animals in order to move up in the pack order.
The only thing you can really do is attach a leash to her and be prepared to give the leash a short tug and low toned NO anytime she even looks at the older male. In this way, she will learn that you are not going to allow that behavior. You, as the real boss, are the one to stop this behavior. I suggest that you enroll her in an obedience class at a minimum. Before you can get into classes, the following site at the bottom has links to specific basic commands which have pictures so you can see you are doing it correctly.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
If nothing else works, you may have to separate the two dogs to help prevent injury to the older dog. I hope this information is helpful to you.