It's my understanding that if someone dies during the process of committing a felony, then all parties committing the felony can be charged with murder, no matter who pulled the trigger, or did the stabbing, or even if the victim died of a heart attack due to fear and stress.
Leaving that aside even...Van Houten was there, was part of the crime, tried to wipe away evidence. Just as guilty as if she had stabbed a living corpse...
As already pointed out, Manson wasn't even there, didn't take part. He masterminded it, ordered it. The rest carried it out. Just as he's guilty, Van Houten is guilty.