Three people having sex together. One person may be in the middle, or everyone can have sex with everyone else. A threesome can take several forms:
(3) MMF (4) MFF
but some level of kinkiness is inevitable.
A threesome involving two men and one woman. There are several options for who actually has sex with whom:
(1) the woman in the middle;
(2) one of the men in the middle;
(3) everyone has sex with both the others.
Thanks to Lara, I managed to get Toby into bed on Saturday; he went for an MMF.
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.