Suegra

Suegra, of course, is mother-in-law; and suegro is father-in-law. Together, they are your suegros. Surprisingly, suegros can also used for the parents of your boyfriend or girlfriend, and there's no suggestion that the relationship equates with marriage or is even headed in that direction. ¿Ya conociste a los suegros? Have you met your new girlfriend's folks yet?

Son-in-law is yerno and daughter-in-law is nuera. The whole family on the other side, your in-laws, would be your familia política.

Like mamá, suegra is often used with the personal pronoun of the speaker's perspective. For example, a guy might say con mi suegra rather than the expected con tu mamá when talking to his girlfriend.

Pareja

By the way, pareja refers both to the couple as well as to its individual members, married or otherwise. It works for gays as well as straights.

• Una muy linda pareja. A lovely couple.
• Es mi pareja. He's my boyfriend. Note that pareja doesn't inflect for gender.

Pareja also works for partner in the police officer sense: the person you are paired with at work. Partners in a law firm are socios.