Serial Number

You must find your sister's serial number for Photoshop CS6 in order to use the software. Perhaps she kept a record of it somewhere. If you know your sister's Adobe logon and password, you might find it by going to https://www.adobe.com/account.html Click Plans and Payments and in the drop down list click Products.

Otherwise, you can try contacting Adobe support.

The property here is not the installation disc - it's the serial number. That's what you paid for. That serial number was your license, and the license was and still is strictly personal.

There is no way to purchase a valid CS6 serial number now. CS6 hasn't been legitimately sold anywhere for 8 years or so.

Any offer you come across now will be pirated and illegal.