need suggestion

This marriage is unlawful under the Hindu Marriage Act, 1955 for Hindus. If the woman insisting it let her be told that forcing anyone for unlawful act is punishable offence for which she can be prosecuted and punished. Let her go and lodge police complaint that you are refusing for this marriage as it is not valid marriage, she will herself be taken to task by police. The marriage between two person who are within prohibited relationship is null and void under the HMA, unless there is any custom or usage in their family or community and such practice has not discontinued and is unlawful and not against the public policy.

In certain States, as per the custom such marriage is valid.  If she files a case, surrender to her or contest the case on merits.  There is no third way.