I am preparing an academic report on laws regarding incest in India. Hinduism law and both major schools under Islamic law consider sibling marriage void. A Special Marriage Act was introduced to govern the marriage among atheists. However I could not find any information regarding consanguineous couples.
See footnote 9 from your Wikipedia link to the Special Marriage Act:
Necessary Conditions For A Marriage
The following conditions are necessary :
- That the parties are not within the degrees of prohibited relationship. However where a custom governing at least one of the parties permits a marriage between them, such marriage may be solemnized nonwithstanding that they are within the degrees of prohibited relationship as follows:
degrees of prohibited relationship
A man and any of the person mentioned in part I of the first schedule and a woman and any of the person mentioned in Part II of the said schedule are with in the degrees of prohibited relationship.
(a) relationship by half or uterine blood as well as by fool [sic] blood;
(b) illegitimate blood relationship as well as legitimate;
(c) relationship by adoption as well as by blood; and all terms of relationship in this Act shell be construed accordingly.
''Full blood'' and ''half blood''- Two person are said to be related to each other by full blood when they are descended from the common ancestor but by different wives.
''Uterine blood''- Two persons are said to be related to each other by uterine blood when they are descended from a common ancestress but by different husbands.
''Ancester'' includes the father and ''ancestress'' the mother.
Parts 1 and 2 of the First Schedule provide a comprehensive list of the prohibited relationships but are too voluminous to quote here.