Dear Sahil
My opinion on your queries are as under:-
1. Can divorce petition (first motion and second motion) is valid if my wife sister sign the papers?
Opinion:- yes, the petitions will be valid.
2. Can my wife later come from US and challenege the terms on which agreement is done?
Opinion:- No, She can not, if the Power of Attorney is Valid.
3. I will be paying alimony on my wife name (draft) so if she later dont come for quashing the FIR after taking money can i recover money later and is it possible to quash FIR on basis of MOU signed of judge will ask her to present or consider her sister power of attorney?
Opinion:- It will be better to retain some amount and pay before the High Court at the time of Quashing. otherwise, you have right to recover the same as per MOU's terms and conditions.
4. What other things should i keep in mind before paying money or signing the papers of divorce so she cant challenge it later?
Opinion:- MOU should be strong and all the terms and conditions should be mentioned.
5. What can be done if she is not willing to come or not able to come?
Opinion:- Fighting the case on merit is only an option
Feel Free to Call