DayInsure is good, but it is better and cheaper if you can find an insurer that will put your niece on your sister's policy. Bear in mind, this addition is relatively cheap whilst she is a learner. The instant she passes her test, it will go through the roof. Personally, I would not even bother putting her on the policy unless your sister can instruct on how to drive to the same level as an instructor. Your niece may pick up on bad driving habits from her. The cost involved would be much better invested in driving lessons with an instructor.