Yes, you'll have to get additional books. Kaplan is the weakest out of the major prep companies in every category, but the worst part is your lack of practice passages. Your cousin took Kaplan's course which gave him access to their online practice material. This is vital. You must take tons of practice passages to prepare for the MCAT.
One of the big reasons BR is popular is because it's tops for content review in most categories AND it contains lots of practice passages.
I'd spend the money on getting the best resources available to give yourself the best chance at doing well. To help bring down the cost, you could try to buy your books from someone. The popular books usually pop up in the For Sale forum.