Just my own experience here, but I had an old dog growing up with a similar problem. She had just the general issues that a really old dog would have, but nothing that impacted her quality of life too much. But then she ended to going blind and deaf around the same time, and she would wander around the house barking like crazy until somebody reached out to her. I think she calmed down because she knew she wasn't alone, I think she was just scared. After a while, one of our other dogs actually started to help her with that. He would walk beside her, with his body touching hers, and she'd calm down. The way your cat calms down when you reach out to her is why I was thinking of it like this.

You mentioned that you lost your cats sister recently. Did her yowling start or get worse after that happened? Because maybe your other cat was what had been keeping her calm, and maybe without that connection she doesn't feel as secure and safe now that she's losing some of her senses. It's just a thought based on my own personal experience