This should be the first thing you tell your mother or Aunt Tilly [tm].
If you do the occasional shopping, email and Facebook usage you only really need to know one password; your email account. The others can be stored in your browser/app or reset if you ever forget. Having to do a password reset before doing your "once-a-year" ordering of photo-books is a minor inconvenience compared to having to remember loads of different passwords or worse; using the same password for all sites.
Teach Aunt Tilly [tm] the typical password-reset procedure and tell her that she doesn't have to remember these passwords, so there's no need for the password to be simple.Shopping sites really should move away from using passwords anyway. They can store a token in your browser and perform a reset using your email address if you're using a browser without the token. They can also do periodic resets of the token.
Just make sure that Aunt Tilly [tm] knows that there is one password that needs to be GOOD and she needs some way of remembering it; her email account. Having access to your email account would give criminals many great ways of screwing you over, since they can reset nearly all your passwords that way.
If she really can't remember a complicated password, then writing it down on a piece of paper in her house is much less likely to cause her trouble than using "mathilda" or "whiskers" as her password.