I had clients once who's children had all sorts of mental and physical problems, and tended to die early. The doctors and mental health didn't know what was going on. I finally did some digging and found out they were half brother and sister! Of course, they still didn't see what the big deal was. Mind boggling.
Are you aware that this is a totally ambiguous (antriguous?) statement?
Who is "they"?
The parents?
The children?
All of them?