Hi
kreel11 ..
I am not sure till what date you can postpone your exam and what is the deadline of the school you are targeting. As a general advise, book only when you are sure to reach your desired score. Your target is 720, so you need to get consistent 720 (+-10) scores to be sure that you will give the similar performance in actual test. Remember, the mocks are different from real exam. Replicate the future exam environment in home/office/library. You should decide the time of exam, snacks you are going to take, break timings, section priority, etc. Do the mocks in similar fashion as you are going to be in real exam.
Also, if you are near to the deadline, it would be best that you talk with your sister about the situation and how important is for you. I am sure she will understand and will not become a distraction for you. I hope it helps. All the best for your preparation and upcoming exam.