Memorializing a deceased best man.

Maybe reserve a seat for him in the front row, with a picture frame of him or of him and his brother together. Or if the guest list is small, and there are no seats, his parents could hold the picture frame.

Especially since the wedding will be limited to close family who are well aware of this tragic loss, I'd leave it up to them to decide if/how they want to address it. I can't imagine there is much chance his brother's absence will go un-noticed. As MOH, I'd think it best if you let your sister know you're willing to do anything you can to help, but then follow the groom and his family's lead. Condolences to your extended family on this terrible loss.

It's very kind of you to think of, but I think the groom and his family need to decide their level of comfort. My dad died before my wedding and I would have been devastated had someone tried to do more to memorialize him than I did - because it was difficult enough for him not to be there. I see the empty chair idea a lot. If we had done that I wouldn't have made it through the ceremony without ugly crying and needing a box of tissues.

I think your intentions are good, but agree with other posters that it may be too much too soon for his family. You can definitely let your sister know of your willingness to do so, but let her talk to her fiancé about what may be appropriate or not for the grief that they're feeling. Some would love to have an empty seat with a frame or other remembrance, and for some it would be ripping open a fresh wound.

Condolences to everyone involved and I hope they're able to have a wonderful wedding even if it's missing someone very important!