From the Editors of The Daily Cat
Bringing an older female cat into a home with an established male is comparable to someone suddenly throwing you into an arranged marriage that you never asked for. Both cats could feel threatened and probably want their privacy back.
Normally, I would advise you to bring home the new cat -- your sister’s pet -- in a closed carrier, which Videl could explore while the older female feels safe. You could still try that, but it may not work as effectively since the two cats are already cohabitating. Each cat should have its own dish, litter box, bed and toys so they’re not in competition for these necessities. Both cats should also be fixed.
Over time, Videl and the other cat may come to accept each other as part of the territory. When that happens, and your sister’s cat learns that Videl isn’t out to get her, she should relax quite a bit. But just as not all people can live together in perfect harmony, not all cats can get along with each other, particularly when they are adults set in their ways. Try to be patient and to make their lives as comfortable as possible during the adjustment period.