She's not being a brat she was just stressed and probably depressed from losing someone she bonded with!! She has not bonded with the other chin and may or may not after some time. I mean, her cagemate/close friend she spent every moment with just passed away so she felt lonely and sad. Chins are pretty smart, and don't just get along with everyone. I would try seperating them and having their cage side by side to get used to each other first.
