Please consider getting in touch with him. Don't worry about what other relatives wlll think, it really doesn't concern them. What about your Mom? In my experience people facing the end of their lifes like to tie up "loose ends" so they can go in peace. Also your brother has a right to know his Mother. My brother was told his bio Father was in prison. I assured him he wasn't.
Please don't wait any longer, precious time is being wasted and your brother has a right to know who is family is.
Keep in touch. Best wishes