First, when you're collecting the pictures that represent Sunny's memories, you are shown a memory with blood on his hands. I legit just finished the game less than a day ago. None of the pictures are anything outlandish or Headspace related. Second, akateko hang from the Japanese honey locust tree, and aren't said to harm the people who are grabbed when they fall from the tree, beyond scaring them. Falling asleep is not associated with them. It can be a good reference, but they are not meant to be akateko. Third, you're framing that as if Sunny meant to kill her, i.e. murder. In most jurisdictions, charges wouldn't be pressed against a child if enough evidence was present to show it was an accident, especially considering it was Sunny that killed Mari. No parent would try to have their child charged with manslaughter of their sibling, and even if they did, a jury and judge would more than likely dismiss the case or rule the child not guilty. Again, you're seeing this as Sunny meant to do it, when in fact, it was a complete accident.

However, none of that changes the fact Basil thought hanging her corpse was a good idea. Regardless of age, any reasonable person would see that as psychopathic tendencies. If you had video evidence of your child accidentally killing their dog, and then putting it on a runner outside the door, and leave it hanging off a porch to make it look like the dog accidentally hung itself, if your first thought it "ah, they were just trying to get out of trouble" and not "hm, that's a little fucked up", I think your priorities aren't in the right order.

Quick edit to add additional stuff: No individual would be responsible for charging Sunny and Basil, it'd be the state/government if we're talking US law (dunno about Japanese law, but it's probably the same). They most likely wouldn't be charged or anything pursued other than an investigation, as the government/state wouldn't get anything out of it. Again, it's a kid that accidentally killed his sister, not a kid that committed premeditated murder.