I'm sorry to hear about your sister's cat, that must have been very unpleasant.
Some animals do increase in population through normal processes of range extension (species' ranges are always changing, needn't have anything to do with people); and a number of others have become commoner (or even move into new areas) because they do WELL by human habitation, not because they are being pushed out of their natural habitat.
Coyotes have been moving eastward for a long time now and increasing in total population in the East. Heck, possums have been moving northward (they have just recently started living in the Toronto area in any number and I am confident they will go from rare to reasonably common here before I'm white-haired). Etcetera.
Deer, grey squirrels, possums, raccoons, and arguably coyotes are all good examples of species that *benefit* from human habitation. They have fewer predators and much more food available around people. Look at what's happened with white tailed deer in the urban/suburban Northeast - there are zillions, mostly because backyards and parks are great, predator-free deer habitat.
With bears and bobcats I am not sure whether it is overall so clear, but certainly in some areas bear populations increase because of human activities (well, til people get serious about removing them by one means or another). I don't know much about bobcat population biology.
Anyhow, point is, it's not always that humans have pushed them out of their natural habitats -- in many cases, it's just that they like inhabited areas and actually reproduce quite well there.
But I am not going to say "if we don't like it we have only ourselves to blame". If your sister had lived there 150 years ago, you can BET she'd have lost bunches of cats to predators. And remember that not all of these species' range changes are due to human activities - to some extent, things just DO shift around and fluctuate in numbers.
Personally I think it's cool, in a lot of ways, that some of the big mammalian fauna is hanging in there so well -- although no, I do not really want bears in my garbage, it's distressing when pets that go outside get killed, and most of all I feel bad for all the coyotes/bears/bobcats/whatever that get road killed or killed as nuisances.
JM$0.02,
Pat