How do I avoid my sisters foot disease?
I have to concur with colethky. Avoid wearing your sister's socks and shoes. Also, some preventive measures you can try include keeping your feet clean and dry, avoiding prolonged moist environments, removing shoes and allowing the feet skin to "breathe," avoiding walking barefoot even while inside your house, and avoiding soaking and using the nail tools that your sister have used. If you suspect that you already acquired athlete's foot, better address it immediately by applying anti-fungal treatment. There are many over the counter remedies that you can purchase. So far, one of the most effective that I know of is ZeroFungus cream. Or, you can also settle with home remedies like tea tree oil and garlic.
Don't share socks or shoes. After she takes a shower make sure you clean where her feet touch. Other than that just don't touch her feet with yours. My husband has athletes foot and I have never gotten it from him.