I agree that it's hard to measure. The word "brotherhood" has certain connotations for me. The first result on Google for "European Brotherhood" confirmed my feelings, but maybe doesn't speak for the majority.
Well, you chose the word. I understood you to mean people having a common identity as a European, in addition to their many other identities (dad, poet, Belgian, whatever). Which I would say is a common thing.
If you were referring to membership of the neo nazi organisation/web shop European Brotherhood, which is a thing that apparently exists, I don't think you need to worry about pan-European-nationalism becoming a mass phenomenon (as opposed to separatist nationalism, which is and has always been a mass phenomenon).
I was referring to "If I believe that all Europeans are brothers" in the parent comment. I have not experienced that sentiment. I certainly never experienced pan-European nationalism until I came across that website.
If you were referring to membership of the neo nazi organisation/web shop European Brotherhood, which is a thing that apparently exists, I don't think you need to worry about pan-European-nationalism becoming a mass phenomenon (as opposed to separatist nationalism, which is and has always been a mass phenomenon).