Birx brother Daniel died in a small plane crash with his wife in 2000.

"Birx’s two brothers also had successful careers in science and engineering. Donald L. Birx, an engineer, is president of Plymouth State University in New Hampshire. Daniel Birx, a research scientist, died in a small plane crash with his wife in 2000."

https://www.inquirer.com/news/nation-world/deborah-birx-white-house-coronavirus-task-force-trump-carlisle-20200330.html

"Daniel and Karen Birx were killed in the crash of their aircraft near Taft, California on March 27, 2000. The cause of the accident was never determined by the NTSB."

https://www.findagrave.com/memorial/70839447/daniel-l-birx