I agree, in anything but a very windy/narrow road with no long straights, the cup would have no chance. Its just that braking hard on a bend in a car weighing the wrong side of 1700Kg and placing the car correctly for the next bend is VERY difficult and requires twice the skill required to do it in a car thats nearly half the weight. So unless brother-in-law is a race driver, hed probably scare himself witless and back off. I speak from personal experience having driven a heavy 375bhp car for a few years. I definitely go faster on country lanes in the cup because I wasnt good enough (or stupid enough) to throw about 1.7 tonnes of metal the way I throw the 950Kg cup around.
The same analogy holds true with an Elise or a caterham which would leave a cup for dead on the same B roads despite having much smaller bhp.