Does my cat miss her brother?

Their old owners had the mama cat and she had a litter of 5 kittens. 3 died and that left them behind. When they were 2 months old, the owners moved away and took the mama cat. The owners were my neighbors, and so, we took the kittens in. They couldn't be separated! When they were a year old, we moved. They always played together and we loved them both! One day, when they were two, Tony was put and he got hit by a car :'( I was extremely sad since he was like my baby. The night he died, I saw his sister(holly) sniffing around the house as if she was looking for something. They were outside/inside cats, now holly rarely goes out. She is always at my side, I love her but before she was never really attached to anyone. Does she miss Tony? I think she does, because they were like best friends.