Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
It seems that you are the second or third customer from India with a similar question and where a mother was jailed for false charges so a relative could get control over the child. With the document you have for PoA, they would have to jail your whole family, you, mother, your husband, your son, in order for one of you to not be able to care for the child.
I would suggest though, because I had another customer with this issue whose sister had her arrested on false charges and took her baby and it took her months to get the child back from the Indian courts, that if at all possible you consider getting your son and daughter in law out of the country as soon as possible because if this other family makes bogus charges against your entire family and everyone goes to jail, as they are the maternal grandparents they could claim a right to temporary custody over the child and it will take time after proving the charges are false to go through the Indian court system to get the child back.
Of course, then if you prove the charges are false you could sue them for filing false charges, but that process could take years for you to ever see any justice.
I am afraid that because the parents are not US Citizens only the child, it makes it difficult for the parents to get help from the US consulate and they will not take custody of the child I am afraid, they are not set up for that. I would suggest that it would be best you get your son and daughter in law out of the country with the child so her parents cannot intervene.
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This is NOT the practice of law nor is it legal advice to you, it is merely educational information for you to use to seek out a licensed attorney in your state to get actual legal advice from them. Please use sites such as http://www.martindale.com or http://www.lexmundi.com or http://www.hg.org to find a local attorney to get actual legal advice in all matters.