Oh, Ann, I am so sorry to hear this about beautiful Tina. As long as Tina is not suffering and is happy, I do not think it is at all cruel keep her. But to make sure of that, I think, if she were mine, I would try some/all of the good advice given here so far, especially a vet evalution.
Sending love and positive thoughts, and hopes that you will find an answer that will enable you to help your sweet one and keep with you for a good while yet.

Hugs
Chris