The idea that two loving parents sexually assaulted their beloved child's corpse to prevent a little kid from going to prison defies all logic. They're rich and white, and powerfully connected. No kid from that background is going to get sent to prison or foster care for an accident. It also just doesn't make any sense that they'd be so cavalier about Burke the next morning (not paying attention to where he was, letting him go away with friends without any concern about him saying anything) if the whole thing was a cover up to protect him. He was so young, would you really trust a kid that young who'd just been through an unimaginably violent experience to keep their mouth shut?
The autopsy (the full non-redacted autopsy results that were released much later on, not the original, redacted autopsy results that the Ramseys' own experts commented on to the press) showed conclusively that she'd been routinely sexually abused over the course of a long period of time. Statistically that points to an adult male relative.
It seems unlikely that the person who'd been molesting her, wasn't the same person as the one who killed her.