My favorite site for such things, behardware, has a wonderful overall rating system that averages around 20 benchmarks into an overall rating. The last test they did which involved those two chips gave the core 2 9550 a score of 124. The Pentium D 950, which is actually a slight step up from yours was rated a 42. By their rating, the 9550 is 3 times faster than the Pentium D.
As for whether he is trying to cheat you or not, such a conclusion is premature. There are other factors to consider. For example, he could have a home built machine with a high end PSU, high end board upgradable to core 2, high end GPU, much larger hard drive, DVD or blue ray drive/burner, etc, while your machine could be a $300 OEM machine whos only advantage is the existing cpu.
However, I would not make the trade. The information we do have suggest such a trade would heavily favor your brother. Even if the above scenario is true and things are more even than they appear, there is little evidence you would benefit more from your brothers machine than your current setup.