Another good trick is to treat a babysitting job like an event. Get a movie, order some pizza and let the kids stay up a little later. The novelty of these little extras should raise them all out of their normal experience and into "babysitting mode," which is where you want them. The sittees will be much more likely to listen to both you and the sibling you have put in charge. Another trick that will make babysitting seem special is to send the new adult to CPR, first aid and Heimlich maneuver classes, or even babysitting training classes. You can find a breakout of the best classes at SitterCity.
In rare cases, you might watch your kids together and realize that the sibling you had in mind as a babysitter is just unsuited for the job. If you suspect that this is the case, get an outside opinion from a teacher or friend who knows your child. Certain kids are just wholly unsuited to sitting, and if you and your consulted party think that this is the case, don't force it.
Make it professional
Certain elements should accompany every babysitting job, including emergency contact numbers, medicine instructions, a house safety tour, bedtime hours and special babysitting rules. Instead of letting these go with a sibling babysitting, because they are a part of your family, make sure to stress these elements as much as possible. Put basic emergency numbers on the fridge or fill out an interactive babysitter cheat sheet, found online. Show them how to dispense medication and review bath time and bedtime rules to make sure they have it all down. Give them a house safety tour that includes all fire extinguishers, emergency exits and child safety locks — you'd be surprised how few kids notice these in their own homes. Finally, make a list of babysitting rules that can be placed in a prominent place, and include on it lots of information about what is and is not included on the job. Telephone and computer usage rules, friend rules, rules about how to deal with strangers and any light housework should be included.