Second cousin marraiges in india

Under Hindu Marriage Act marriage between sapindas is prohibited. Cousins separated up to 5 generations on the father's side and 3 generations on the mother's side are considered sapindas. The individual is considered as generation one for this purpose. In the case mentioned by you the marriage is prohibited as they will be sapindas. Special Marriages Act is mainly intended for marriages between those who may not be blood-relations. But if a couple eligible for Hindu Marriage opts for Special Marriage I see no reason why the same rules as for Hindu Marriage should not apply.

Thank you for your valuable feedback, agreed this relationship is not acceptable under Hindu Law and Special Marraige Act

When you get married in a manner that you think that yours was a valid marriage why should anyone challenge your marriage?  So long as no one challenges you will continue as husband and wife. Someone may challenge your marriage in a court of law if one has to gain something by that the other person is not your spouse or by that your children are not your legitimate children. It could be some other reason. I was married when I joined service and I gave the name of my wife as my wife. My employer did not question it. My wife all the time got all the benefits an employee's wife was entitled to. Suppose another woman had come and claimed the benefits which my wife was entitled to, then she, my existing wife or I will have to go about substantiating the claim of validity of the marriage. If you were a US visa holder and you wanted to apply for visa for your wife, the US consulate would be satisfied with your marriage certificate. If in your country a brother can marry his sister under law the US authorities would not question you. But if someone challenges with a claim, the US authorities may look into it. If you marry your paternal second cousin nothing will happen unless someone challenges. If someone should challenge there must be a reason to challenge. Then the court would decide.

As I said earlier the relatives who are not in agreement to this relationship, can they challenge this in court of law and make the relationship void on any grounds that I am missing here. Has there been any earlier cases like these and was the ruling in favour of the couple or the family

There are any number of cases where validity of marriages on account of prohibited degrees of relationships have been questioned. Unless specifics are given it is difficult to answer such questions. Say what exactly is your problem.