I'm thinking you're going to receive recommendations like take an old photo and photo shop him in....which I have seen done before, and quite frankly, I find that more disturbing.
Honey, your family's loss is significant, and your mother will probably never 'get over it.' But Life does move on, we do march forward.
Perhaps you can take a family picture in a favorite location of your brothers? What comes to mind when you think of him? A sport? Certain music or restaurant? Favorite color - then all wear that?
I think it's really wonderful of you to bring the family picture up again. I would ask your own family this same question here. And you can all figure out together how to honor him in the group picture?
Honestly, you ARE still a family....just minus one. That does not mean you don't get to live and enjoy your lives now. Doing this will help her realize that her son also lived, and not just died.