
I like the idea of animals having souls outside the heavenly structure. I tend to agree with this view more than your brother-in-law's view. But then, I'm a bit of an eccentric.

Near the end of his life, contemplating the reunion with his beloved dog, the poet and mystic William Blake was asked by a friend, "Do you actually believe there are dogs in Heaven?" to which Blake replied, "How could it be Heaven without them?"
My sentiments exactly.