These are some of the words you will need to explain how the members of a family are related to one another. There are a few tricky bits. One is that the terminology for in-laws works differently as described below. A second thing to watch out for is that cousins are frequently called brothers and sisters.
свёкор—the husband’s father, the wife’s father-in-law
свекровь—the husband’s mother, the wife’s mother-in-law
тесть—the wife’s father, the husband’s father-in-law
тёща—the wife’s mother, the husband’s mother in law
двоюродный брат—first cousin (male)
двоюродная сестра—first cousin (female)