Is marriage legal or not

@datt k Yes she might have sign somewhere, but can't we prove she was at home on 26th dec. 
Sir what we will do, if we go for divorce it will take more than 1 and half year. And we want to close this case  as soon as possible. As my sister is 28 and this year she will be 29 and after one and half year, it's long time. We want to close the case and make her settle down.
That's why we void the marriage.

You have stated

Yes she might have sign somewhere, but can't we prove she was at home on 26th dec.

Why are u not sure?

In case you want useful suggestion, please come out with full facts as to why they showed marriage date as 26 dec.  what is the motive?

Marriage with underage is voidable and can be terminated.  Other fraud cases will,have to be proven in the court of law.