They were not involved in the decision-making process to have another child whatsoever, so they shouldn’t have to deal with the consequences of that decision. Children should be able to BE children when they are young. They shouldn’t have to take on adult responsibilities like child-rearing unless they make that decision themselves one day. The effects of delegating child-rearing to the older kids are nearly always negative, in my experience. Many of the people I meet who came from large families want absolutely nothing to do with children once they are adults because they spent their entire childhoods taking care of a gaggle of other siblings.
They are stressed out and cranky and, more often than not, the parents will take this out on their children. This is usually unintentional, but people are a lot less patient when they are overworked and their tempers are a lot shorter. Children, obviously, require a lot of patience, so this is not a good combination. Less-than-stellar parents may even come to resent their children for making them have to work so hard, which is even worse for the children to have to deal with. In very large families, it is common for the older kids to take care of the younger ones