Chinese traditional society has always attached importance to blood relatives and clans, which constitute our big family and small family. So what are blood relatives? In short, it is blood relationship. Blood relatives include direct blood relatives and collateral blood relatives. Direct blood relatives refer to those who have the relationship between birth and death. People who are directly related by blood become direct relatives, including parents, grandparents, maternal grandparents, children, grandchildren, grandchildren, etc. It is worth noting that in addition to natural lineal relatives, lineal relatives by blood also include lineal relatives created by law, such as adoptive parents and adopted children, adoptive grandparents and adopted grandchildren, and stepparents and stepchildren who have foster relationships are lineal relatives. Collateral relatives are relatives with indirect blood relationship, that is, relatives who are not direct relatives but share the same origin with themselves by blood, such as brothers and sisters.
 Although in real life, collateral relatives by blood may be more closely related than direct relatives by blood, for example, someone has a better relationship with his sister than his parents, in many laws, direct relatives by blood and collateral relatives by blood still have different rights and obligations. For example, direct blood relatives are not allowed to marry, but collateral blood relatives of more than three generations can. For another example, when the heir has parents, children and spouse, siblings cannot inherit.
 Legal basis:
 Article 7 of the Marriage Law of the People's Republic of China
 Marriage is prohibited under any of the following circumstances:
 （1） Direct blood relatives and collateral blood relatives within three generations;
 （2） Suffering from a disease that is medically considered unfit for marriage.
 Article 10 of the Inheritance Law of the People's Republic of China
 The estate shall be inherited in the following order: first, spouse, children and parents. The second order: brothers and sisters, grandparents, maternal grandparents. After the beginning of succession, the successor in the first order shall inherit, and the successor in the second order shall not inherit. If there is no successor in the first order, the successor in the second order shall inherit.