British Shorthair kittens & cats in Mocksville, North Carolina

I've had Grendel since she was born from a neighborhood stray that we were taking care of. She is mostly an indoor cat, but does enjoy wandering around the yard, but she is mostly satisfied being indoors all the time. She has recently become more aggressive towards another cat that she has been living with for several years. She has always been less than happy with sharing the house with another cat, but has done so relatively amicably her entire life. She expresses her frustration with the other cat but chasing her and spraying whenever we're all in the same room. I was hoping this was a phase she would get past, but it has been occurring for weeks now. Grendel is 12 years old. She likes to sleep on furniture, the floor, and on my lap. She is litter box trained. She has been deaf for a couple years since the vet treated her for an ear infection. She also is missing her top eye teeth but can still eat dry food from a bowl.