Brandy Kills Ray J and Princess Love's Wedding First Dance SongI!

Ray J's big sis, Brandy, brought the best gift to his wedding -- her absolute A game when it came time to serenade the bride and groom with an Etta James classic.

We got this video of Princess Love and Ray hitting the floor for their first dance, and apologies to the happy couple -- but when you hear Brandy's voice, it's clear she stole the show for these few minutes.

Check it out ... hard to give her less than a 10. Even Ray J and Princess stopped staring at each other, and opted to watch her performance instead.