How to get your parents to let you get dogs?

My sister and I really want to get 2 dogs. We already have 2 dogs at home. Our dad has already said no. I am 17 years old and my sister is 10 years old. We have offered to pay for the the cost to take care of the dogs. We have also offered to get good grades and do choirs around the house without being asked. We need some advice on what to do or say to convince our dad. What is good advice?