No, those first two behaviors are NEVER appropriate. These behaviors raise red flags for history of abuse.

This may sound harsh, but they are sexually assaulting the younger children. These children (the older one/s) should not be left alone with younger children. I don't know your relationship to the children, but you need to get help for all of these children. You should call child protective services for help. If you are the parent, and you allow this behavior to continue then you are responsible. If you are not the parent, then you need to get help for these children also by calling CPS. (If you are in the US- I'm sure other countries have similar systems). Don't let this behavior continue, you can't change what has already happened, but you can keep it from continuing. These type of behaviors don't stop on their own, they just progress to worse behaviors.