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.
"You and Johnny are such good friends How do you even know him?"
"Oh Hes Toms best mate so hes my Best friend-in-law"
When you become best-friends with someone through marriage.
My best friend married Maya's best friend, so now we are best friend-in law's
The best friend of your spouse.
My wife's best friend Karisti is my best friend in law.
when your best friend is dating someone the boyfriend/girlfriend becomes your best-friend-in-law
person Dada ad person Sham are dating and person Sham and person Yaya are best friends,Dada and Yaya are now best-friend-in-law