A brother in law is the brother of your spouse, but sometimes the relationship can become as close as that between two blood brothers. An in law is someone who you are related to by virtue of marriage, and there is no blood connection between these family members. A brother in law can be a great source of joy or a big source of stress and conflict, depending on the person. If you have a brother in law treat them with courtesy and kindness, the same way you would your relatives by blood.