Next time she comes round and asks about them
Just play along. Call them Dave and Jemma. Then, in the next sentence, call them Graham and Susan. Next time, James and Margeret. Repeat ad nauseum.
(rnuk http://flickr.com/rnuk, Fri 12 Jan 2007, 16:17, archived)

Although that's not entirely unheard of.
My parents have on various occasions in the past referred to me as "James" (my brother) and "Jess" (the dog), and many other things.
( Moon Girl Technologies horrendous beanbag, Fri 12 Jan 2007, 16:20, archived)