Trump is "sure" the family wants to reopen the investigation. How is he sure? Did he communicate with any of them and if so, did they tell him they want to have the death reinvestigated? From what I read, the family accepted the medical examiner's findings that this was not foul play. The medical examiner's findings were reviewed by other pathologists who agreed with those findings. And LEO has said several times, it won't reinvestigate because there is nothing to reinvestigate.
These conspiracy theories are nothing more than speculation by people with a lot of time on their hands, but no proof of their claims. Apparently, those people include Trump.