IMO they do not work there the one on the left will hit someones head and the other just not right and IMO family pics are best hung in non public spaces like a hallway going to the bedrooms not in a LR. How about you look for a nice piece of art more horizontal tand vertical to work with the vent on the wall then hang those shelves in a hallway or somehwer else.

The floating shelves I've seen usually have accent decor pieces on them. I have 5 floating shelves in my bedroom and they all have accent decor pieces on them. No photos of people. I second Patricia Colwell Consulting on the placement of family photos.

What is the depth of the shelves? Patricia has a good point about head bumping. Also, the shelves aren't placed well and the frames are too small. The higher shelf is too high up and separated from the sofa and the other shelf. Kind of like it's floating up and away. Some art over the sofa would be nice. Something that's about 3/4 the length of the sofa and hung about 6 inches above it.

Shallow shelves can work for art over a sofa. If your shelves were shallow, you could bring the higher one down in line with the other and place just a little closer together. Lean some art on the shelves such as in this photo

Perhaps shift the bottom shelf up further and add bigger art in different sizes on shelf and some other art between shelves to bring it together better in balance as well as disguising the vent

"Small nick-knacks" and "clean, uniform look" are oxymorons! Think large and unified design like a big piece of art you love.