Answer:
   
   There are a number of prohibited relationships in Judaism. The Torah
   sets forth a list, and any community cannot, by custom, violate a
   Torah prohibition. For example, a man cannot marry certain close blood
   relatives, the ex-wives of certain close blood relatives, a woman who
   has not been validly divorced from her previous husband, the daughter
   or granddaughter of his ex-wife, or the sister of his ex-wife during
   the ex-wife's lifetime. These are all enumerated in Leviticus 18.
   These include that one cannot:
     * Have sexual relations with one's mother (Leviticus 18:7).
     * Have sexual relations with one's father (Leviticus 18:7).
     * Have sexual relations with one's father's wife (Leviticus 18:8).
     * Have sexual relations with one's sister (Leviticus 18:9).
     * Have sexual relations with one's father's wife's daughter.
     * Have sexual relations with one's son's daughter (Leviticus 18:10).
     * Have sexual relations with one's daughter's daughter (Leviticus
       18:10).
     * Have sexual relations with one's daughter (this is not explicitly
       in the Torah but is inferred from other explicit commands that
       would include it).
     * Have sexual relations with one's fathers sister (Leviticus 18:12).
     * Have sexual relations with one's mother's sister (Leviticus
       18:13).
     * Have sexual relations with one's father's brothers wife (Leviticus
       18:14).
     * Have sexual relations with one's father's brother (Leviticus
       18:14).
     * Have sexual relations with one's son's wife (Leviticus 18:15).
     * Have sexual relations with one's brother's wife (Leviticus 18:16).
     * Have sexual relations with one's wife's daughter (Leviticus
       18:17).
     * Have sexual relations with the daughter of one's wife's son
       (Leviticus 18:17).
     * Have sexual relations with the daughter of one's wife's daughter
       (Leviticus 18:17).
     * Have sexual relations with one's wife's sister (Leviticus 18:18).