Marriage annulment

You can file petition for annulment of marriage under voidable marriages under Sec. 12 of Hindu Marriage Act.  Following are some of examples of consent by force or fraud on which marriage can be annulled.  

(i)---Concealment of factum of annulment of first marriage at the time of second marriage.

(ii)---Non disclosure that party getting married to petitioner is partially disabled because of polio in the childhood.

(iii)---Threat to commit murder or to commit suicide.

(iv)---Impersonation at the time of selection for marriage i.e. person shown was different than person actually married to the petitioner.

(v)---Respondent allegedly suffering from heart disease from childhood.

(vi)---Concealment of fact that the respondent was suffering from tuberculosis

(vii)---Non disclosure of alleged illicit relations of respondent with third person.

You case comes under point (vii).  In my opinion, you have to give one chance to your husband to change his attitude, because having illicit relationship in foreign country are common.  If you are much serious about this issue, you can file petition for annulment of marriage under Sec. 12 of Hindu Marriage Act.  You can show the evidence you have with you i.e. voice recordings, email conversations, witnesses etc.  You have to file petition within one year from the date knowledge (knowing the fact of illicit relationship).  After one year your right to file petition will cease as it is voidable marriage.

the marriage should be under SPECIAL MARRIAGE ACT

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The step what your sister has taken is a right step.  It is very correct that she moved in a right direction by filing a petition for annulment of her marriage on the grounds of fraud played by her husband by suppressing the fact of his previous marriage with another female which was material to this marriage wit your sister.   Ask her not to budge to any pressure or threats from any quarter and not to withdraw her case, she can very well include the name of the previous wife (if known) and also affirm before the court that the marriage was not consummated. Be in touch with your lawyer constantly.