There is no one English word to match all the obligations and anxieties due to your machatunim (pronounced maa cha TUH nim- the "ch" is a gutteral sound as in "J.S. BaCH"). While the English "in-law" relates a person to the relatives of his or her spouse, the Yiddish "machatunim" is a Yiddish word relating someone to the relatives of the spouse, or even potential spouse, or his or her offspring. For instance:
A --- B C --- D
If E and F are married, or, more pressingly, are considering getting married, then A and B are the machatunim of C and D, and C and D are the machatunim of A and B. "Machatunim" is a plural noun. The singular male noun is "machatin" and the singular female noun is "machateynes". It's a loaded word, and tends to imply people who you've got to somehow impress. Usage: "Get out the good china, the machatunim are coming to dinner!"