Modified a bit
In your case the only contention can be for the existence of a prevailing custom which allows such marriage....Customs vary...I am not much aware about the Punjabi customary practices...
The custom should have prevailed for atleast more than 17 - 20 years...
cus·tom/ˈkəstəm/
Noun: |
A traditional and widely accepted way of behaving or doing something that is specific to a particular society, place, or time. |
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A prevailing practice in a community like the gandarbh form of marriage in maharashtra...
or even places in south allow marriages like in the case cited by you i.e in the same family...
Narrowing it down.....contesting will be difficult...
Regards