Everyone who's confused: it's not actually that complicated, and none of it was a dream. It was just a flash-back inside a flash-forward used to establish a parallel between how the six were after the last arc, and how they were after the wedding.
And honestly, I'm satisfied. I expected him to pick out each of the sisters in the final quintuplets game and to give their respective relations closure, and I'm happy that's basically what happened! For in essence the final chapter, it may seem strange that it focused so much on the other sisters, but it doesn't invalidate anything. Fuutaro and Yotsuba are together. They're together, partly, because of the other sisters, and to the very end, the relationship between him and the quintuplets, and the quintuplets' bond with each other are what this story was built on.
This chapter doesn't seem to be listed as the END though, so I wonder what we'll get to see in an epilogue?