Atheist getting islamic marriage ceremony for the sake of tradition by parents?

Assalamuailakum, Is this sinful? My parents want to do an islamic marriage ceremony for my athiest brother (hes 28yo) and his girlfriend. They do not believe in Allah swt but my parents say as muslim themselves it is their duty to get them married islamically. This doesn't make sense to me as I feel like it is disrespecting the Islamic marriage tradition. The two people doing it need to have faith in Allah swt to be able for it to mean anything, they would have to convert to Islam right? Otherwise its just an act and its better for them to just get a civil marriage done in the eyes of the Law. Am I correct or are my parents allowed to make them do a "pretend" Islamic marriage ceremony for traditions sake? They also say doing the Islamic ceremony even if he and his girlfriend do not have faith is a good thing because he might come back to faith through it? And therefore they are doing their duty as Muslims. My brother being an atheist doesn't care either way about the religious aspect and will just do it because it means nothing to him and he mostly just wants the traditional wedding experience.