Hi pj, long time no see. :-)
I have to wonder what she meant by "Be My Valentine", also.
Are you telling us everything? Have your letters gotten romantic?
If she were writing to me, I would think that she just wanted me to be a special friend. And, it sounds like you may already be that special friend. I would say "go with it" and respond by saying that you would like her to be your Valentine, also. Every lady likes to think that she is special to someone, and for a prisoner, I would think that the feeling of being special, for someone, is magnified many times over. Can't you let her enjoy that feeling? :-)
Live simply, love generously, care deeply, speak kindly.
Cowards are cruel, but the brave love mercy.
"The purpose of life is to contribute, in some way, to making things better" Robert F. Kennedy