What is Affinity?

Legal Definition
In law and in cultural anthropology, affinity, as distinguished from consanguinity (blood relationship), is the kinship relationship that is created or exists between two or more people as a result of somebody's marriage. It is the relationship which each party to a marriage has to the relations of the other partner to the marriage; but does not cover the marital relationship of the parties to the marriage themselves. Though laws vary considerably, affinity does not always cease with the death of one of the marriage partners through whom affinity is traced, nor with the divorce of the marriage partners. In addition to kinship by marriage, "affinity" can sometimes also include kinship by adoption and step relationship.

Under the law, such relatives by marriage are known as affines. More commonly, they are known as "in-laws", as affinity is usually signified by adding "-in-law" to a degree of kinship.

In law, affinity may be relevant in relation to prohibitions on incestuous sexual relations and in relation to whether particular couples are prohibited from marrying. Which relationships are prohibited vary from country to country, and have varied over time. In some countries, especially in the past the prohibited relationships were based on religious laws. (See, for example, Affinity under canon law.)
-- Wikipedia
Legal Definition
A connexion formed by marriage, which places the husband in the same degree of nominal propinquity to the relations of the wife, as that in which she herself stands towards them, and gives to the wife the same reciprocal connexion with the relations of the husband. It is used in contradistinction to consanguinity. (q. v.) It is no real kindred.

2. Affinity or alliance is very different from kindred. Kindred are relations. by blood; affinity is the tie which exists between one of the spouses with the kindred of the other; thus, the relations, of my wife, her brothers, her sisters, her uncles, are allied to me by affinity, and my brothers, sistors, &c., are allied in the same way to my wife. But my brother and the sister of my wife are not allied by the ties of affinity: This will appear by the following paradigms

My wife's father ---| | | | | -----------------| | | | |-- are all allied to me. Ego ----- My Wife 0 My wife's sister ---| | | 0 My wife's niece ---| My wife's father, ---| My Father | |My brother | | |and my wife's | | |sister are |---------------| |----------| |not allied | | | | |to each other My brother Ego ---- My wife, My wife's sister, |

3. A person cannot, by legal succession, receive an inheritance from a relation by affinity; neither does it extend to the nearest relations of husband and wife, so as to create a mutual relation between them. The degrees of affinity are computed in the same way as those of consanguinity. See Pothier, Traite du Mariage, part 3, ch. 3, art. 2, and see 5 M. R. 296; Inst. 1, 10, 6; Dig. 38, 10, 4, 3; 1 Phillim. R. 210; S. C. 1 Eng. Eccl. R. 72; article Marriage.
-- Bouviers Law Dictionary
Legal Definition
At common law. Relationship by marriage between the husband and the blood relations of the wife, and between the wife and the blood relations of the husband. 1 Bl. Comm. 434; Solinger v. Earle, 45 N. Y. Super. Ct. 80; Tegarden v. Phillips (Ind. App.) 39 N. E. 212. Affinity is distinguished into three kinds:

(1) Direct, or that subsisting between the husband and his wife's relations by blood, or between the wife and the husband's relations by blood ;

(2) secondary, or that which subsists between the husband and his wife's relations by marriage ,

(3) collateral, or that which subsists between the husband and the relations of his wife's relations. Wharton. In the civil law. The connection which arises by marriage between each person of the married pair and the kindred of the other. Mackeld. Rom. Law, ยง 147; Poy-dras v. Livingston, 5 Mart. O. S. (La.) 295. A husband is related by affinity to all the consanguinei of his wife, and vice versa, the wife to the husband's consanguinei; for the husband and wife being considered one flesh, those who are related to the one by blood are related to the other by affinity. Gib. Cod. 412; 1 Bl. Comm. 435. In a larger sense, consanguinity or kindred. Co. Litt. 157a. See Quasi affinity.
-- Black's Law Dictionary
Legal Definition
Tie between a husband and the blood relations of the wife, and between a wife and the blood relations of the husband but not between the blood relations of either and those of the other. See 6 L. R. A. 713; also 79 Am. St. Rep. 195.
-- Ballentine's Law Dictionary