With brittle bone disease and kidney failure, is this fatal?

Not necessarily,: If kidney failure means on dialysis it is worse than if not on dialysis. At any rate need to control your phosphate levels and take your meds as prescribed to control you bone disease so no hip fracture will occur which will increase mortality. Control phosphate by no dairy products and control bean/ legume intake. Also, no cola including dr. Pepper, but root beer is okay.