I assume this is UK based? I have to say, it is not unusual for such firms to be somewhat dilatory.
Are this firm a firm of solicitors or are they are firm of Trustees (ie ABC Trustees Ltd). I only ask because it depends on how to work their complaints procedure if it comes to that.
In terms of you not wanting them to be executor, unfortunately, you cannot force them to renounce (ie resign) their position. if your brother has appointed you and them jointly, they have every right to obtain the Grant as much as you do. However, you can ask them nicely if they would consider renouncing their right to the Grant and if they agree to do so, you can then obtain the Grant on your own.
In the meantime, I would call them and not give them a rollicking, but find out what the delay is explaining that you are anxious to commence the probate process.