I would ask my client if I could answer brother's questions. In fact, brother most likely has tried to get answers from your client, and we all know he can't explain it like you can. Your client may have told brother to call you, who knows? I would be happy to answer questions about a return I prepared, as I'm sure you are, Terry. I would also ask client if you can give a copy of the Form 1041 to brother. That way, brother's preparer can look it over. I would want to know where the info on my K-1 came from, too. Possibly just having a copy of the Form 1041 would remedy the whole situation.
Regardless, I'd talk to client first and get a feel for whether there is discord here, or just a simple desire to understand something that affects my tax return.