I had a sister who passed away last year when she was thirteen. I want a good memorial song for her to play while showing her photos on a projector in my wedding.
Sorry for your loss. My wife went through this process a few years after we married when her brother died suddenly. She was putting together a slideshow as well. She went with a quasi-religious song even though her brother had probably never heard it and probably wouldn't have liked it, it just seemed fitting.
Having said that, there is this song https://m.youtube.com/watch?v=A7ry4cx6HfY which although from a metal band (that I don't really like), is actually quite emotionally rich, written for their drummer who had passed away. If the music style is a little too harsh, you can find acoustic covers online.
Good luck with your new life with your wife.