"Not they're not! Unlike cheating, they are both aware of their partner's sexual activities. They also set up rules to avoid jealousy and heart break."
A polyamory relationship in which two or more people are in a romantic relationship with the same person without being in a romantic relationship with each other
Kitchen table is a style of polyamory that places emphasis on family-style connections among the people involved in a network, whether they're romantically involved with each other or not. The name comes from the idea that everyone involved would be comfortable gathering around the kitchen table for a meal.
A form of relationship involving more than two people. The relationships may be symmetrical (like a triangle), or something more complex. The relationships may also be open or closed, straight or gay, or a mixture.
A purely heterosexual form of polyamorous marriage was once practiced among the Tibetans: each man had many wives, and each woman had many husbands. This raises the possible of your brother also being your husband-in-law-in-law.