Sudden change in behavior

Our two cats are siblings that we have had since they were kittens.  Maud had to go to the vet for dental work and was gone all day.  When I brought her home and let her out of the carrier, her sister Molly began hissing and growling at her, as if she did not recognize her.  I thought she might be reacting to the smell of the vet office, but two days later and she still hisses menacingly. I am at a loss about how to fix the problem.  Any help is welcome.

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