I'm so sorry. What a hard thing for all of you.
Did you bring Joey home with you? To bury him? I now make sure to have a little ceremony in our back yard and bring the other animals out to say goodbye. Because they DO miss each other but animals understand death if they are given a chance to know it happened. Without the viewing of the body I too have found the remaining critter, sometimes not the same species, will look for the missing one, sometimes for weeks. I've not tried it but maybe burying a blanket or something Bella associates with Joey would help her. Something that smells like him.
Personally I would not get another cat for Bella (and yourself, it would YOUR cat) too soon. Bella needs to grieve too, I think. A child would know the difference, a dog would too, I believe.
I'm so sorry. My condolences to you and your family.