Hello, it seems that she might be missing the other cat, but, is she spayed? You never said if she is fixed, or how old she is? Any time behavior changes a vet visit is in order, vocalizing may indicate physical discomfort. Cats also vocalize for attention and if you give it to her when she talks, then she'll keep doing it. If she is griefing from loss, (cats don't like their routine messed with), try to keep the schedule the same, and give extra cuddle and play time, it should take care of self.If your cat is new to your home or has just gone through a change (move, new person/animal in the household, person moved out)and has just started her talkative behavior, be patient. This may be happening due to the transition and will stop on its own if the behavior is not encouraged. Remember, even scolding can be perceived by your cat as attention, and thus encourage the behavior. Hope this helps, get to a vet for check up first.