Are you talking about one with ME/CFS and only one more with another illness? Or, is the one person, the one with ME/CFS? Sorry, brain fog...

I didn't "know" I had hEDS until diagnosed a few years ago, and I'm 42. I went to a Rheumatologist for joint pain. During the exam, he said "you are really flexible". He did an evaluation and I scored a 9/9 on the Beighton test. That, along with other clinical signs, made the diagnosis. Always, growing up, and now, my older sister and I were/are super flexible. I just did not know there was an actual name for it. After the diagnosis, and speaking to other family members, is when I found out that my grandmother is also the same way.
This is why these questions need to be asked, there may be a connection, but sometimes, it might not be something that jumps out at us, especially if certain things are "just part of who we are"
Also, certain genes die out over time (as we share 50% of genes from each parent, 25% from each grandparent etc.)
But, sometimes, genes pop up, after not being seen in a family for awhile. Like the random red headed cousin, where everyone else has brown hair. I am the only one in my family with curly hair. Genes are complex
Some of my health issues (including hEDS) were present before I developed ME/CFS (at 17) and some trickled in later on, as a consequence of it.
I got ME/CFS after a bad case of mono. I shared a soda with a friend. Why is it, that the person that I directly got mono from, had a mild case and completely recovered and I had a severe case and never got better?
Some women, develop ME after childbirth. Women have babies every day.
Life happens everyday, stress, illness...but, why do some of our bodies, around 1% of the world's population, go on to develop ME/CFS? That is the million dollar question