Your in-laws are the family you get through marriage, not through birth. People most often mean the father and mother of your husband or wife, but the word can cover brothers, sisters, and the wider family too. “The in-laws are coming this weekend” usually means those parents, and sometimes the whole visiting side. It is everyday English, a little informal, and almost always plural when you mean the group. For one person you say mother-in-law, father-in-law, sister-in-law, and so on. Though actually the “law” here is the old idea of being related by the law of marriage, not by blood — that is why they are “in-laws” and not “in-bloods.” Some families use it warmly; some use it with a small sigh.