If your sister was driving at the time, your options are to name the driver, or commit the s. 172 offence by doing nothing. The former *might* win you 6 points for permitting, whereas the latter *will* win you 6 points. We cannot advise you to commit an offence, even an offence of omission, and there would seem to be little benefit in committing an offence to replace a likelihood of 6 points with an almost certainty of 6 points. If you were tempted to be a little "creative", there is a real chance of a custodial sentence if you get caught.
However, *if* after retracing your steps you remember that you were actually driving and that your belief that your sister was driving was based more on the presumption that you would avoid liability by naming a foreign driver than it was on fact, then again you should name the driver.