Wearing someone else's eyeglasses can make your eyesight worse, but this is not permanent. If you just wear them for several seconds, it is OK. However, if you wear them for a long time, this will cause problems. When you are wearing glasses with incorrect prescription, your eyes are forced to adapt to that incorrect prescription, which causes eyestrain and headache. If you wear them in long term, your eyes will not focus correctly without eyeglasses any more. I suggest you not wear others' eyeglasses and get your own prescription eyeglasses.
In general, wearing other people's eyeglasses is bad to our eyes, and it may lead to diminution of vision. It is known to us that people's prescription is different form each other. So we had better wear a pair of glasses that fit us properly. Insevere condition, you may feel dizzy and even get a headache when you wear other's glasses for a long time. Even if you have the same eye prescription to your sister, i still suggest you wear your own glasses.
If you have the same prescription with your sister, you will not make the eyesight get more serious if the size of the eyeglasses is suitable. However you should not exchange the contact lenses between yourself which will get your eyes get infection. You should not do this. It is very dangerous for your eyes health. In order to improve your eye sight, you should eat more food with vitamin C which will moisture your eyes.
As you say, your prescription is the same, I think you can wear her glasses as long as you won't feel uncomfortable. If you feel uncomfortable wearing her glasses over one week, then you should get your own prescription glasses.