Should we be freaked out or supportive?

I am recently married, and each of our best friends have "fallen in love". These are long time friends and seeing them together is weird at times, my wife has gone as far as to say their fling reminds her of a brother sister scenario. My first thought was to be supportive, however the thought of it going all wrong will always lead one of our best friends to be alienated. One thing is for sure there's no going back to the way it was before. Are we over-reacting, or has it gone too far?