Also, I bought a book called Self-Scoring IQ Test for Children by Victor Serebriakoff and had my son try when he was 8. He got 94 problems right out of 100. How well does this kind of test correlate to a test administered by a professional?
This test does not have the same ceiling in scoring and is more heavily weighted in the verbal skills in which the gifted often score high. Professionals often use it to test a person we know is gifted to determine just how gifted she is. A gifted person scoring 160+ has different learning needs than a person in the 130 range.
Self-scoring IQ tests on the popular market are considered only an indicator of intelligence level. The results may be contaminated by the environment in which the test was taken (for example, your sister told you the answer!).
The information contained in both your questions, however, tells me your son must be very gifted.