Bismihi ta'āla
Have you expressed your concern about this to them? Have you spoken about not only how it is haram, but also how it will deprive them of barakah and tawfiq in their lives? Try your best to politely explain all of this, and remind them.
If they still insist on having haram elements in their wedding, I would recommend you attend only the essential parts of the wedding, so as not to create conflict and future problems for yourself, as he is your brother. You can then excuse yourself, once there are haram things. If anyone objects, or gets upset, that is for them. You cannot obey or please someone at the cost of disobeying and displeasing Allah ta'ala.
And Allah knows best