Those of you that have older kids and younger kids how do you keep the older kids from treating?

the younger kids like they are babies and doing everything for them. My oldest is 11 and my twins are 3.. The problem I have is the twins are the whiniest by far of all my kids. After pondering the question for a while I have noticed a pattern.. They whine to their older siblings who eventually get tired of their whining and do whatever they want for them. They will also go to the older siblings for things they know they should not be doing etc.. So those of you who have successfully managed this how do you teach the older kids that you cant just give the younger kids what they want all the time?