I had almost the same problem, when I switched back from 17s to stock. My car couldn't go over 35mph!!! I was shocked and lost (cause I could do XXXmph with 17"s). Also, when it topped out at 35mph, the traction control light came on. Does your sister's car do it? If yes... well my problem was is that I had bigger rear (195/75/14) tires than front (195/65/14). So I switched them, now the car runs perfectly. 17"s were better though...
