Cats die. It happens. You mourn and get another one. It's called life.
Sorry to be heartless, but I've had two pets that have died, and I do miss them a lot, but you have to move on at some point.
My dad claims he is allergic to cats, but I don't believe him bc my grandmother had cats and we visited he frequently, and I had a cat for two years, and I remember one time coming home one day to find my father watching football and holding and petting my cat. Oh, and he somehow isn't allergic to dogs 🤷🏻♀️
But since your brother is most likely actually allergic, just try to keep the cat away from him. Or like Emily said, let it be more of an outside cat.