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If it doesn't cause damage and your Saint doesn't object, you may not need to do anything.
Since it could start to cause damage, spraying a taste deterrent on the area of the Saint that the Chichi is biting at might stop the behavior. Bitter apple and Yuck are two brands that are commonly available.
Let me know if you have follow up questions.
She may be smelling something abnormal particularly if he has been feeling poorly recently. Did the vet do any blood work (complete blood count, blood chemistry screen)? Is she leaking any urine (common in older females)?
His leg may shake because of pain. If so, it can be treated with one of the non steroidal antiinflammatory drugs (NSAIDs) available for dogs. OTC human drugs are not a good idea unless prescribed by your vet because dogs metabolize them differently than humans do and toxic reactions are common.
I have a lot of respect for the abilities of dogs. They have senses that humans do not and move in worlds we do not understand. She may be responding to odor or some other cue, but her biting the Saint is more likely to be a response to something than an attempt to alleviate pain.