Making sunset portraits is a photography skill unto itself. When shooting on a beach at sunset the conditions are rarely the same. Thus, not only do you have the usual variables to deal with when shooting on location, and not only is there a ticking clock with the sun getting closer and closer to the horizon, there is also the unknown of how the sun will effect your shot according to its strength.
If you find the setting sun is overpowering your portrait, then a good photography tip to use is to block most or all of the brightest part of the sun with the subject. This will reduce lens flare too.
Of course unless you want the subjects to be silhouettes, you will need a speedlight or two off camera to light them up. The sun is then used as a backlight source complementing your portrait instead of dominating it if the subjects were not blocking any of the strong light coming from the setting sun.