Sometimes (too frequent for my liking) when questioned "Why did you do this 100% prohibited or simply boneheaded thing", my 6- and 8-year-old kids' response is "he/she - pointing at the sibling - told me to do it".
At that, BOTH of them fully and immediately agree that they BOTH knew at the time that they were explicitly prohibited from doing whatever was done.
Is this sort of behavior dangerous? To me, this projects to the future as being more susceptible to peer pressure in negative situations, AND as a major sign that parental opinion and authority is held pretty low ("Didn't we discuss yesterday that when mother tells you one thing and your sister the opposite, you listen to your mother?" "Yes". "So, why did you do it when she asked"? "I donno")
What are good strategies to curb that pattern? I would estimate that they get up to no good at least 2-3 times more often when they are together than when they are alone.
The interesting thing is that it's not ALWAYS younger or older following the other's bad ideas. They both do it.