What Breed Is My Dog?

I bought this dog for my girlfriend and I have no idea what it is. The mom was a brindle Pit Bull Lab mix, kinda short, but really stocky. Hazel is my dog and she's got a brindle coat. My friends bought the sisters too. Both are the same color, only one is the color of the mom. They have long bodies and are also pretty stocky. She towers over them in height, but not body length. The lady I got the dogs from doesn't know what the dad is. Help? Thanks!