Affinal kin are individuals who are related to you by marriage. Unlike blood relatives, affinal relations are based upon a legality or contract. Although they are considered to be members of your family, the kinship tie can be broken if the marriage dissolves, rendering you no longer related to the individual in question.
Stepparents and Stepsiblings
In the event one of your biological parents marries, his spouse becomes your stepparent. For example, if your biological father gets married, his wife is then your stepmother. Likewise, any children of the new spouse become your stepbrothers and stepsisters -- not to be confused with half-siblings, who share one biological parent.
If you marry an individual who already has children, those children become your affinal kin. For example, if a man marries a woman with a young son, the man then becomes the stepfather to the child, and the child is the man's stepson. Although you do not share any blood, entering the marriage makes you legally related to your new spouse's children.
When you enter into a marriage, your spouse's family also become your affinal kin, in this case in-laws. Your spouse's father becomes your father-in-law, and you become his daughter- or son-in-law. In-law relationships are not restricted to the nuclear family alone, but include the entire family. For example, your spouse's uncle becomes your uncle-in-law, and you become his niece- or nephew-in-law.
Nieces and Nephews
Nieces and nephews refer to your siblings' children, and these are blood relatives. For example if your brother has a daughter, that child is your niece. But children of your spouse's siblings are your nieces and nephews as well. These nieces and nephews are your affinal kin.