First off, I think it is great that the 4 and 5 year old want to help with their sister. That shows you have some caring children.
Second, I do think that you need to establish some boundaries and rules. Children are a gift, not sinful creatures to be disciplined. However, they do need guidelines and structure. Some children need more than others. A good combination of setting up rules and consequences and sticking to it and positive reinforcement will help you. Reward them for following the rules. If you only focus on the bad behavior, they will see that as the way to get your attention.
I think it might help to establish some routines and give them some "chores." For instance, I have a 3 1/2 year old son and a 2 year old daughter. My son needs more structure and can obviously do more than my daughter. We were struggling with some tantrums and not listening - so we did a list of rules, with pictures that he has to follow. My daughter has to follow them too (not all as she is too young for some). I made some of the rules tasks that my son could do and give him a sense of accomplishment - and my daughter does them too. For example, they are to put their plates in the kitchen once they are finished with dinner. They put their dirty clothes in the hamper in their rooms.
This makes the rules not all bad and scary.
I hope this helps.