When I nannied my little brother and sister and they would bicker or not get along, I'd give them a chore or activity that requires working together. Once I made them write and act out a play about consequences and siblings who didn't get along. Once I had them do the dishes together, one was the washer, the other the drier. I found that those punishments did much more for growing their relationship than just plain old taking stuff away.
Nickel - mama to boys D 09 and F 10 and my girl H 12
#4 due in October!