Must have been,
Undisclosed illness:
https://www.msn.com/en-us/news/politics/donald-trumps-brother-robert-trump-dies-of-undisclosed-illness/ar-BB180BfL
Officials did not immediately release a cause of death.:
https://kstp.com/national/robert-trump-the-presidentrsquos-younger-brother-dead-at-71/5828229/
I find reported the cause of death nowhere