Both of my sisters (currently mid-30s) have had their lives on pause for over 10 years due to chronic Lyme disease because doctors in Mexico hadn't ever even heard of it. It took 4 years of pain for the first of them to be diagnosed. Not sure when, if ever, they'll be cured because when you don't treat Lyme disease within a few months of infection, it digs in and is incredibly difficult to kill.
Unfortunately nobody wants to say it to your face, but among females, you can never rule out social contagion when confronted with extremely coincidental outcomes like this.
Or, it could be a genetic condition they both have being misdiagnosed as chronic Lyme via internet research.