What Breed Is My Dog?

I recently bought a puppy from unreliable owners. When they brought the puppies out they said the mom, she looked full Pit Bull, but she was very small and not so stocky like a normal Pit Bull. They did not have the dad on sight, but they had a picture of him and he looked like a Pit Bull mix because he was bridle with an oddly shaped face. I have a feeling that my dog is a Pit Bull mix, but I just can't figure out what the mix part actually is.

In the litter she came in she had two brothers. One looked like her with different colors and the other looks nothing like her, it looks more like a Pit Bull (it's mom). My dog is the black one, her brother who we also bought is the light tan color, and the boy that went to a different home is the darker brown one. The mom is the light peach color. She is the only dog with a collar on. If anyone has any idea what mix my black and tan dogs are it would be very much appreciated.