marriage

If you are Hindu see whether the Gotras are one and the same.  If the gotras are one and the same, marriages are not permitted as per Hindu custom, as they are related to one Ancient Rishi.

Waht do you mean by "forfather"?

Need clarity on your query 

Which religion do you profess i.e., Hindu Muslim, Christian ?

Do you belong to any such Caste/ Tribe where such marriage is allowed ?

Generally such marriage is void.

Nothing can be presumed by experts with such limited facts posted by you.

It is advisable to consult a local prudent lawyer for better appreciation of facts, professional guidance and proceeding.

Until the relationship do not fall under the list of prohibited relationship to marry or coming under sapinda and the caste customs to not allow such marriages to be legally recognised, the marriage conducted so may be termed as voidable.

It cannot be said that it is null and void until it is proved beyond doubt that this relationship is prohibited to get married to each other through court of law.