09-25-2015, 04:57 AM
The whole "apostrophe S" thing is a greyish area when dealing with names, there's no strict rule. Both are used and acceptable; there's both a St. James's Park and St James' Park. I personally write it with it, so I'd write "Nibbs's". See the first answer to this and the second-to-last paragraph of the second answer to this.