If you can see better with your sister's glasses, it suggests that you ought to arrange to have an eye test and possibly get glasses of your own.
Wearing somebody elses glasses won't make you need glasses (that story is total hog-wash!). Think about it logically, if looking through lenses forced your eyes to go bad, everybody that frequently uses such things as microscopes, telescopes, binoculars, rifle scopes, etc would ultimately mess up their eyes. The worst you could do from wearing her old glasses is get headaches. Although it is unlikely for 2 people to have the exact same prescription, if the glasses are "close" to what you need, they will be better than no glasses at all.