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Most airlines allow pets in carriers to travel in the cabin with their owners, however the carrier needs to be of certain dimensions. They usually require that the carrier is able to fit on the floor, under the seat in front of you, when you're on the plane. Your cat is a large one, and if her carrier is of a large size, they may say it is too big; however, it would not be fair to your cat to have her travel in a smaller travel box.
In addition, some airlines will not allow pets to travel in the cabin on some international flights. I checked American Airlines for you, just as an example, because they fly to Sao Paolo, and they do not
allow pets to travel in the cabin on trips to Brazil. Here is further information: http://www.aa.com/i18n/travelInformation/specialAssistance/travelingWithPets.jsp?anchorLocation=DirectURL&title=pets#PetsTravelingInTheCabin
What I suggest is that you make a list of all the airlines you are considering, based on availability of flights on the dates you need and on price, and ask what their regulations are, re: traveling with a cat in the cabin. If you would like to tell me the airlines you're interested in, I would be happy to research them for you.
I agree that your cat should not travel in the cargo hold, but if you cannot take her in the cabin due to her size and the size of her carrier, consider leaving her with a close friend or relative who is experienced with cats and already knows her, while you're away.
I hope all will work out well for you and your kitty and wish you safe travel!
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