16. Prohibited degrees of relationship.
No marriage shall be valid—
(a) where either party shall be directly descended from the other; or
(b) where the female shall be sister of the male either by the full or the half-blood, or the daughter of his brother or of his sister by the full or the half-blood, or a descendant from either of them, or daughter of his wife by another father, or his son's or grandson's or father's or grandfather's widow; or
(c) where the male shall be brother of the female either by the full or the half-blood, or the son of her brother or sister by the full or the half-blood, or a descendant from, either of them, or the son of her husband by another mother, or her deceased daughter's or granddaughter's or mother's or grandmother's husband.
17. Marriage of persons within prohibited degrees of relationship an offence.
Any marriage or cohabitation between parties standing towards each other in any of the above-enumerated degrees of relationship shall be deemed to be an offence, and shall be punishable with imprisonment, simple or rigorous, for any period not exceeding one year.
http://srilankalaw.lk/Volume-V/marriage-registration-ordinance.html