Linking the parents to the crime-scene, when the crime-scene is their own house, doesn't require any forensic evidence. Both parents are inherently linked to the crime scene.
Linking them to the crime itself is another story. An example of a real link would have been a scratch on a parent's face or arms that was noticed by detectives. Or JBR's blood found on a parents article of clothing. Instead, most links are contrived and seem to be motivated by sensationalism. "Oh they're rich and bought their way out of it" is an RDI rationalization that doesn't explain why LE never even bothered to book them. OJ, RB, and MJ were all booked, remember?
The R's didn't get booked because there was no evidence linking them to the crime itself. Obvously they're llinked to the crime scene. Big difference.
They ARE linked to the crime scene. PRs jacket fibers (from the jacket she wore the night JBR was killed) were found entwined IN the garrotte knot. Also on the mouth side of the tape. The tape was removed by JR before she was brought upstairs and before PR "supposedly" saw her. JR shirt fibers were found on her pubic area. You can explain away those fibers on everything else, but those three places are SPECIFIC TO THE CRIME ITSELF and links them to the crime.
The Rs didn't get booked because the DA refused to allow it. Period.