I am an Australian Citizen and I wanted to know whether I will have any problem in giving my E3 visa interview in India.

Is there any particular reason why you would want to have it in India? The interview generally would be in your own country, in this case Australia.

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Not if your actual residence is in the US. You are aboard ship as part of your employment, with no intention of abandoning your Permanent...

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Citizen application on a work based Green Card and previous annulled marriage

You were asked about the marriage in case they found evidence of fraud. I would leave it alone unless anything came up that worries you.

Applying for mothers visa to come to the States with sibling?

Only US citizens can petition for their parents so I assume you are such. This is called an Immediate Relative petition and, unfortunately, does...

Received an RFE on marriage certificate and new medical examinations aside from my husband. Do I send info requested separately?

You should both comply with your respective RFE's as soon as possible, and separately.

Can both (mother and sister) could apply for my US immigration simultaneously?

They could both file petitions for you. Through your mother you would be F3, through your sister, F4. They are both at least 12 years away from...

You should not assume the worst but still should consider hiring an attorney.

Can I cancel my application to change my visa status from F-2 to F-1?

Yes, in writing and with tracking and delivery confirmation.

Will I be getting a positive result from USCIS for my N400 after this wait based on the details provided below?

No way to predict what the result will be. You will just have to wait and see.

How should I apply for my immigration? Please advise

You would be an F3 through your mother or an F4 through your brother. Either way you are looking at more than a 12 year wait for your priority date...