Had my daughters capsule in the back of my old GTS-t several times, with no problems at all, but it was easier to use the side mounts rather that the central one for the teather. I did have a 4 door, but that really is not the point here...
Actually, a 2 door car is stronger than the 4 door version of the same vehicle. You have all the structural metal near the rear seats, which in a 4 door, is the rear door...
The other point is that you are actually less likely to have the accident in the first place.. By owning a Skyline, you show that you actually enjoy cars, and therefore look after it better than most, get it serviced and make sure that the tyres are safe. You also have brakes and a chassis of much higher quality than certainly the Holden equivalent..
You enjoy the car, and spending time with you Sis and Nephew.. As for the pretend Brother in Law, leave him to play with his Daewoos... after all, that is what most Holdens are..