In medicine and law, relationships are measured in a matter of degrees. The more closely related you are to someone, the lower the degree. The medical definition of a first-degree relative is fixed, while legal definitions vary by jurisdiction and also by the specific legal matter.
Medically, the degree of relationship is established by the number of shared genes. Relatives who share 50 percent of your genes are your first-degree relatives. This includes your biological parents and children, as well as your full siblings. Half-siblings, who share approximately 25 percent of your genes, are second-degree relatives.
In some legal contexts, your spouse and adopted children are considered first-degree relatives. Although the relationship is defined as one of affinity rather than consanguinity, the legal obligations and protections are often identical to those involving biological first-degree relatives. If you have any doubt about the legal definition of your relationship with a particular relative, seek the advice of a qualified attorney.