I agree do tons of research on the animal and make sure that YOU really want one and are able to care for everything it needs (food, supplies, expensive vet appts. Everything) before even asking her for one, as pets are not cheap and most parents end up paying for everything because the kid gets bored or they just let them get one too young.
If you are unable to pay for the animal on your own and do need her financial help, then maybe work out a deal where you trade house work or yard work for her money help with the pet.
It is possible though, I convinced my mom to let me get my first pet at the age of 12 (I did have to get a job babysitting first though to ensure she wouldnt have to pay for anything unless it really was necessary)
I wrote an essay type thing about the animal I was interested in, where it would be housed so it was out of their way, reasons why it made a good pet, reasons why I chose this animal to have, I added up an average of cost and told them how I was going to pay for it all. Eventually they let me get my first pet AFTER I had raised the money for it plus some extra for future items.
Of course cute pictures definitely help, good luck!
Also just a tip, in my experience the more I bugged and bugged my parents about it the more they wanted to not let me have a pet, so just come up with a plan first and give her time.
