Hi, I'd like to help you with your question.
It sounds like there might be a few different things going on with your niece.
One, she could be depressed from her mother's death. Not everyone reacts the same way when depressed. For some people, they stop being responsible and act out. This could be the case with your niece.
Two, she was raised without boundaries and as a result, does what she wants to rather than what is responsible to do. At her age, that is difficult to correct since she is legally an adult. However, that does not mean it is impossible.
Three, it may be a combination of the two other reasons.
Your idea of taking away the car is good. She needs consequences. However, if you have not talked with her about depression and seeing a counselor, you might want to consider that first. If she is depressed, she may be willing to see someone. There are campus counselors available on most college campuses. She could try there or talk with her family doctor about a referral.
If you decide to take away the car, talk with her first. Let her know you are upset about her behavior and that you will take the car away if she does not improve. Give her a list of steps you want her to take to prove she can be responsible.
Keep in mind that providing her with all the materials and tutors is great, but it will not help her if she is not motivated to learn and take advantage of your generosity. She needs to learn responsibility for anything to have an impact.
There is also a book that may help. It is called Grow Up!: How Taking Responsibility Can Make You a Happy Adult by Frank Pittman. It is available on Amazon.com or your local library may have it.
I hope this helps,