Hmm interesting. Actually don't need 2 versions of LW on same system. What I think has happened is your sister's installation has removed your LW. And because it was installed from her account, you can't uninstall it or much more likely, your uninstaller is now invalid.

The answer is to create a shortcut from her LW installation & send copies to both your account Start menu options & also desktop. I am not sure but she may need to do that for you. But not necessarily. Remember the program folder is a shared folder so to speak. It caters for all users on the computer & is not found in user accounts. So ...

But don't worry, since your LW preferences will be as were before & your incomplete files prefs will also be within your account so both will be safe.