Yes, there's no problem doing that. This means that you will be allowed to drive your sister's car when you need to. If you happen to have an accident while you're using your sister's car, you can claim through her insurance policy because you'll be an additional driver on that one.
One thing worth mentioning is that if you have comprehensive car insurance on your own car, chances are that you may be allowed to drive your sister's car with her permission on a third party basis. This is included in many fully comp policies but you should check yours to make sure that this facility is available on yours too. It might save you from having to pay additional to become a named driver on your sister's car insurance policy.
