Okay I'm going to make allowance for the fact that of course you are insane from the intense responsibility of having a tiny human who's completely dependent on your wife and you 24/7.

Been there, and yes it's different from a cat or even a kitten. Mostly scarier.

For instance a cat's fur and claws mean a plastic bag on their face won't kill them. And they might get into trouble climbing all over the house at night but if they fall they'll land on their feet.

Try to breathe, and remember, the nights may last forever but the weeks fly by.

Appreciate the intensity of your love, but don't begrudge your wife's brother's love for his cats.

The time will come when your kid becomes an adult and you can relax a little bit. And unless you really fucked it up, you'll still be their parents, bound for life by love.

Your wife's brother, who is just trying (failing but trying) to relate, will at some point be grieving the death of those cats, and I hope you'll do a better job of understanding his feelings than he is of yours.