The rule states that:

"You must be able to read (with glasses or contact lenses, if necessary) a car number plate made after 1 September 2001 from 20 metres.

You must also meet the minimum eyesight standard for driving by having a visual acuity of at least decimal 0.5 (6/12) measured on the Snellen scale (with glasses or contact lenses, if necessary) using both eyes together or, if you have sight in one eye only, in that eye.

You must also have an adequate field of vision – your optician can tell you about this and do a test."

If your sister meets these requirements then they're not needed (even if she wears them at other times). Only an optometrist will be able to provide the required evidence.