How horrible!
I know a new puppy is out of the question, but maybe he could get an adult dachshund from a rescue? That way, it could come pre-pottytrained, used to kids, etc. Its such a horrible feeling to not hear any doggy footsteps in the house, and I'm sure they'd like to have the baby grow up with a dog around.
Are your brother and his fiance in the area as well? I had to give up my foster dachshund back to a rescue years ago, and the wonderful lady that took her said that she often had dachshunds in her rescue. I could find her email if you want...
Sorry for the loss of Molly Marie.