Your sister has urine storage defect. Normally the urinary bladder is able to store urine up to 400 to 500 ml at a very low pressure before the bladder filling sensation is felt by a person. At appropriate time bladder outlet relaxes while the bladder muscle contracts to empty the bladder completely. Any defect in the bladder or outlet function may result in bed-wetting. It could be a defect in the bladder muscle or its outlet. She would require to undergo physiological testing of her bladder, urodynamic study, before knowing the cause of her problem. A defect in bladder function, specially the storage pressures, may cause back pressure affecting the kidney function. My major concern is that the untreated bladder defect probably damaged her original kidneys, and may put the transplanted kidney in similar danger too.