Section 5 of the Hindu Marriage Act of 1955 forbids marriage within the prohibited degrees of relationship and between sapindas, close relatives. Both rules apply “unless the custom or usage governing each of them permits of a marriage between the two.”
Section 3 defines custom as a rule observed “continuously and uniformly” for a long time that “has obtained the force of law among Hindus in any local area, tribe, community, group or family.”