"My grandmother has bad hearing."
When using soba, no need to use "watashi no" as soba already implies ones own grandmother and also no need for desu at the end depending on formality.
"Soba" means noodle but it is a common mistake and is sometimes used as a joke. "Sobo" was used in my sentence for grandmother.
It seems it to us, but people are always asking me 目が悪い (me ga warui?), so I think using "warui" is a normal way to express that someone can't hear well or can't see well.
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I think "うちの" should be taught instead of "わたしの" for family. I wouldn't even use "わたしの" for my cat because it's a part of my "home base"