This is one of your girlfriend or wife's sister or sisters. This relationship is one that is contextualized by using fratty language, such as referring to each other as "bro" and using the word "hella" frequently as an adjective in encounters where both parties are involved. The chick-bro-in-law makes your relationship a packaged deal.
Michael: "Hey bro you wanna hit the club up tonight?"
Joel: "No bro I'm getting hella drunk with my girl and chick-bro-in-law tonight, at this fancy restaurant. It's gonna be so bromantic."