My six year-old son has gotten in the habit of copying his little sister. It feels like he is taking her behavior as cues to what is acceptable, but the act of copying itself (and usually amplification) pushes it over the line.
For example, yesterday my daughter was singing quietly in the car, and my son started singing the same song, but much louder, much faster, and with some stated "corrections." First of all, that's upsetting to my daughter to have her song hijacked. It's also quite annoying to everyone else in earshot, including being distracting to the driver.
He doesn't seem to understand why it was okay for his sister to sing, but not for him to copycat. Any ideas for helping him understand?