Oh gods that's horrendous I'm so sorry. Rats are well known for attacking weak animals ( in the wild, but domestic rats still have all the same instincts of course ) and eating them. Quite a few don't realise. They will take young bird chicks if they can get them, or if they fall from the nest ect.
How on earth did your brother's rat get to the rabbit? Why was the rat loose?????
This is awful I can't even begin to know what to say that would offer any comfort, other than that although her death was unpleasant and horrific, she isn't in pain anymore.