If it's who the 'obvious' google search reveals, you've said too much already (and at least one the links that search gives reveals the family name directly).

I suspect the worry is in the other direction. Sure it is not difficult to figure out what the brother's pen name is. And if you know both his names, it is not hard to find connections between them.

The part that popular authors usually want to avoid is making it easy to go from the pen name to the IRL name if you don't have some specific guess or anything. Right now I don't think going in that is particularly easy. If the grandparent however has said the name here it would almost certainly have made it easier to get from the pen name to the IRL name.