I have 7 kids, ages 16, two are 15, 14, 6, 3 and 1. And my oldest are always parenting the younger ones, but that is to be expected if you ask them for thier help, no matter how small the help may be. I do however stay on top of how much "parenting" they do, and always step in when it isn't thier place ie.. punishing them, deciding what they can and can't do, except where it might be harmful. Decide for your family what would be ok and what wouldn't, and set the ground rules.
Know to the "just a second" I do not allow that in my house, if they are in the middle of something they may ask for 5 more minutes and if it isn't done in that time frame they are reprimanded, For example if I ask for the shoes to be picked up and they don't the shoes go in the trash, if I ask you to help with setting the table and it isn't done without me having to ride them, they don't eat dinner, it doesn't take to may times before they get the message. Just remember if they are in the middle of a show, don't ask when it's at the good part wait until the comercial(sp) or warn them during the show that at the comercial you want such and such thing done, if it isn't done during the comercial turn off the T.V.
There is always going to be times were you are going to feel the need to blow your cool, my household is by no means peaceful, the girls either love each other or hate each other and at thier ages they always think they are right, but by having some basic ground rules the amount of time you feel the need to blow your cool can be reduced, sometimes I wish I had " stepford children" and yet I also enjoy the chaos that comes with each individual personality. Don't know if I helpsed any but good luck.
P.S. All my kids loved Narnia includiag the 3 and 6 year olds, it is a fun and sad movie.