I don't know what the etiquette books would say but if you enjoy giving her a gift and it doesn't seem to be a burden to her i.e. she seems embarrassed about not getting you something, then I think it is fine, nice actually.

I used to live in a neighborhood that had a quite a few widows. I make jewelry and I used to give each of them something I had made along with a Christmas card. Most of them seemed to really appreciate it :-)

Generosity is one of the main themes of Christmas. Good for you!