Since it has been 3 weeks I think maybe you should take her to the vet and have her checked out to make sure there is nothing physical going on.  I know you are probably cringing at this suggestion since you had such a bad experience before.  I am hoping that maybe you can find another vet you trust to take her to.  Animals do grieve over losses just like we do and she may be just "looking" for him, not understanding exactly what happened to him.  But again since it has been so long already and I am assuming this just started, there may be something else going on.  Especially since she is elderly herself.  If she checks out ok then you may want to think about getting another kitty for her as a companion.  At that age though it may be a bit more difficult getting her used to another cat.  I am thinking a kitten or young cat would be good for her.   Talk to your fellow pet owners and see who they use for vet services.  Whiskers' death was traumatic for all of you and going to the same vet will bring back hurtful memories, but at the age Brandi is, she really needs good vet care and there really could be something else going on with her.  Just for a trial you may want to let her sleep with you guys, or get her a stuffed animal to cuddle up with until you can be sure about why she is howling.    Keep us posted.  If she is ok healthwise I am sure this mourning period will pass.  It may just take some time and some extra TLC to get her over her loss of her brother.

seems bad to guilty mother