Is there anyone near you, say Amish or Old Order Mennonite, or trainers that have horses that hitch? Someone that can help you with this so that you learn a good method, and are safe?
A Saddlebred barn, STB, Trotters or such would be able to help you, and be able to teach you and your pony and your sister, as if she will be doing the driving? She will need lessons too.
Where are you located, as I have friends that drive, and you might be close enough to let them help you. PM if you aren't comfortable saying on here.
And do you already have the equipment, or are you going to have to come up with all of that too?
Horses make me a better person.