The odd occurence that when you are going out with someone you inherit their best friend as one of your own good friends. Often occurs more that a girl will become very close to her boyfriends best mate than the other way round.
Your bestfriend's husband/wife or girlfriend/boyfriend. So if your bestfriend is cheating then you have 2. Also if your bestfriend has a celebrity crush and says they are married that's also your bestfriend in law.