As others have said, it's a big age difference. But the 12 year old is trying to exercise her growing independence from adults and work her way out of childhood and into adolescence. So she's going to boss around anyone she can. The 6 year old knows that the 12 year old is not a grown up, so she thinks the older child's attitude is the goal. She also thinks it's acceptable, as long as the 12 year old is allowed to get away with it.
I don't think you should tell the older one she's the role model anymore. She's going to take that as "I'm an adult so I can be bossy."
The thing to do with any and all unacceptable behavior is to take away something that child likes. For these girls, those will be different things. For the older, it's probably electronics and free time at the mall/sleepovers, a later bedtime, shopping trips, beach/pool trips and maybe allowance. For the younger one, it's probably TV time, play dates with friends, and things like trips to the beach or pool. Separate them when they get into negative behavior, with as few words as possible - "That's unacceptable. You can spend time in your room until you can be less bossy/more respectful." Then stop. Be specific about what's not allowed, but don't get into a long explanatory lecture about it. Be swift and decisive. Deprive them of your attention as well. Take away immediate privileges, especially for the younger one. Delayed consequences won't be as successful, and they will increase the amount of time the kids spend whining/complaining/negotiating. I didn't put up with the griping though - that increased the consequences.
The point is Privileges X and Y and Z are for kids who are mature and who know how to behave with kindness, not for bossy kids with attitude.