Step 1: Understanding the Concept:
The Hindu Marriage Act, 1955, prohibits marriages between persons who are within the "degrees of prohibited relationship". However, the Act provides an exception for established customs.
Step 2: Detailed Explanation:
The relationship between a man and his sister's daughter falls under the definition of "degrees of prohibited relationship" as defined in Section 3(g) of the Hindu Marriage Act. Specifically, it is covered by the rule that one cannot marry a descendant of one's brother or sister.
Section 5(iv) of the Act states that a condition for a valid Hindu marriage is that "the parties are not within the degrees of prohibited relationship, unless the custom or usage governing each of them permits of a marriage between the two".
This means that such a marriage is generally void. However, if there is a long-established, continuous, and certain custom in the community to which the parties belong that allows such marriages (as is prevalent in some parts of Southern India), then the marriage would be valid.
Court or Panchayat permission is not the deciding factor; the existence of a valid custom is.
Step 3: Final Answer:
The marriage between a man and his sister's daughter, though generally prohibited, is valid if a custom governing the parties allows for it.