Contact lenses can be divided into several types, such as yearly contact lenses, semi-yearly contact lenses, monthly contact lenses, bi-weekly contact lenses, weekly contact lenses, and daily contact lenses as your sister's.
Daily contact lenses are disposable, but other types need to keep using until the deadline of their valid periods. It's pretty dangerous if someone wear old contact lenses.
If you don't want to clean your lenses, you can buy daily contact lenses like your sister.
No, not all contact lenses are disposable. There are different lenses that you can keep using for different length of time. Mostly, there are daily disposable contact lenses and monthly contact lenses. For daily contact lenses, you can just use them for one day and throw them away. But for monthly contact lenses, you can throw them away after one month.
In fact, not all contact lenses are disposable. Generally speaking, there are three kinds of contact lenses,namely daily, monthly and yearly contacts. Disposable and daily contact lenses means you can just wear for one day. If you wear disposable contacts for more than one day, you are more accessible to infection, oxygen permeability and allergies for the cleaning condition for disposable contact is not as good as that of monthly and yearly contacts. Therefore, if you just wear it for one day you can choose disposable contacts. But if you want to make your contacts more than one day, then the monthly and yearly contacts will be your perfect choice.