Kids refuse to say sorry because they can. It is like not going potty or to sleep or not eating. It is something they can control and they use it cause they know it makes the adult mad.

Making the child continuously say sorry doesn't address the issue. Why should she be sorry for protecting what is hers. It is up to you as the adult to keep the younger sibling out of her way either by sectioning off one of them or providing a higher surface for play or closer supervision or giving the older child an appropriate thing to do when her sister comes such as moving around to be between her sibling and the work she is doing so the sister can't touch it. We need to spend more time teaching coping skills and when we do the negative problems disappear because they have a method for handling them.