If you have an opportunity to closely examine the boat, there are a few things that you may look for too answer a few q's. If you look at the helm of the vessel, you will see a drive indicator of some kind. It will be numeric. Write down the last setting or posistion of the drives. I believe numbers go from 0 to 9. In the same or very close location you will see a similar gauge for TABS. Record both of these settings. The speed of the accident will never truly be known but for your personnel info you may get a max (for the day) recall on the GPS speedo. This may give you an idea of the rate of deceleration (from 120 to 70 in 1/4 mile etc. etc.). I imagine that since they over shot the first stop, they pulled back on the sticks relatively hard. This factor could have given your brother a "false" sense that they were actually going at a slower rate than they really were as a result, he thought it was OK to make the turn. People get the same sensation when traveling on a 75mph (90mph ) Thru Way and get off at an exit with a 55 mph road. You don't realize your traveling 65. Your senses are still set for 90mph. The TAB and Drive settings will give clues if the attempted turn was properly "prepared" for or not. When a boat operator is setting up for a hard turn it is common practice to lower the drives a bit in attempt to get more "sticking power" to the transom of the vessel. The TABS don't really have much use in a turn. They are however, used to make the boat plain out faster as well as correct a "leaning" boat. The Tab reading may lend a hand in determining what COULD of happened during the spin. They could in-fact, play a large role in determining the motion of the boat during the spin.
When you look at the boat where are the cracks, holes, rips etc. etc. This will be the most important clue in determining the actual impact of the hit. If you can look at the restraint mounts, if they were in-fact wearing the restraints, you may be able to determine (Bends, holes in the fiberglass etc. etc.) which way they we're thrown. Early posts stated that all the seat backs were bent toward the bow suggesting a strong force came from the transom direction ( my previous post theory). Check the direction of the drives. They were making a turn so the drives should hypothetically (If the force of the spin didn't alter them or rip them off) still be in that direction. If in-fact they are turned in the opposite direction, it may lend clues as to whether or not your brother tried to (had time) correct the boat or it suddenly broke free. If the boat suddenly broke free it probably would have spun in a more even or flowing fashion. If he tried to correct the spin it would affect the area at which the boat impacted. Kinda like "high siding" a motorcycle. I'm not trying to sound "basic", I'm just letting you know the basic functions of different parts of the boat. If you already know..sorry. If you are able to take pics of the boat on land it would really help the oso gang in theorizing on what may have happened. Hope the info helps.