As nutty as RFK, Jr is about some things, he is much saner than the loons currently in power. He actually seems like a normal person compared to them. Sirhan was apprehended at the scene with the literal smoking gun still in his hand. I think some Kennedy family members want the deaths of their loved ones to be about more than a random lunatic. The MLK family has fallen into the same trap. It's unfortunate.
Whatever one thinks of RFK Jr, the forensic evidence strongly indicates that Sirhan wasn’t the only person who fired shots in the kitchen that night. Even Vincent Bugliosi, a former LA prosecutor, acknowledged that the case for conspiracy is strong in RFK Sr’s murder.
The Medical Examiner til this day doesn’t believe Sirhan could have killed RFK Sr.
Obviously, the evidence that there was a conspiracy doesn’t exonerate Sirhan given that he was there with a gun in his hand. And that was basically Kamala Harris’ rationale for denying him a new trial (back when she was a CA Prosecutor).
When Sirhan was originally on trial, his lawyers didn’t challenge the evidence. Their goal was to get him to plead guilty in order to keep him from getting the Death Penalty. But since the 70s, it has been well documented that the LAPD destroyed evidence and that there were too many shots fired for Sirhan to have been the only shooter.