As we all know that cucumbers are really good things, because we can not only eat them, but also use them to make our skin more beautiful. What's more, putting cucumber slice can help in reduce swelling around the eyes and the big dark circles under your eyes. Also, using cucumber slice can lighten the skin. I think those are the reasons why your sister uses them for her eyes.
Hearing this, I want to ask a question, does your sister use it after a long day's hard work? If yes, the puffiness and the tiredness may appear on the eyes. While cucumbers are the most wonderful and natural eye pads. Using them can help reduce the puffiness and tiredness. This may help you understand her behavior. Maybe you can try it too, I believe that you will benefit a lot from keeping on doing so.