How old is Melody? Do the rest of the family let her play with her feet and expect her to understand that she just cannot play with your mom's feet because she is sick? I am diabetic too and it has been quite a trial with Trjegul because he is a biter and I have thin skin. Since I got him I have never had whole skin on my arms. BUT he is getting much better. I spent three months constantly blowing in his face, telling him NO, tapping his nose and even grabbing him by the nape of the neck (he got very agresive at first) to get him to stop. A friend of mine who used to be a vet tech came by quite often to visit and did the same thing. We did not attempt to play with him by hand. We did eventually learn that he likes to chew on fingers without biting and we let him do that. Now a days he is fine and the only reason I have some small wounds is that I will play with him now that he knows better and he gets carried away. At which point its my fault because if I didn't play with him with my hand he wouldn't get carried away.
Suggest that your mom keep a soft toy with a bell or something by her and if she starts to play with her feet she can throw the toy to distract her. Or she can keep a wand toy nearby and use it to distract her. But the rest of the family must also never let the kitten play with thier feet.