Female, age 91 yrs with peripheral neuropathy, feet lost sensations. Will vitamin B12 injections help?

Possibly. B12 deficiency can certainly cause peipheral neuropathy and the treatment would be the replacment of B12 usually by injections. As far as other causes of peripheral neuropathy, B12 most likely will not be helpful. See her physician and check a B12 level if not done already.
Maybe. There are many causes of peripheral neuropathy, and B12 deficiency is only one of them. B12 injections are beneficial if you are deficient (or have low b12). Some adults lose the ability to absorb oral b12. Nonetheless, I recommend a daily b-complex vitamin supplement. Have your B12 level tested 3-6 months later. If your level is less than 400-500, you may benefit from a monthly injection.