I write to ask how to renege on a signed postdoc position in the US, and whether there should be legal consequences. During the embassy interview waiting time, the health of one of my close family members deteriorated and it would be very difficult for me to travel even for one year. I have not applied for the visa yet.
I would guess that most universities would accommodate you in some way if you just ask and explain the situation. There is little benefit to anyone to try to force the issue.
But, you should ask in person if possible (zoom, say) and also ask what your options are. Some places will be able to put off your appointment for a year, others not. But you can ask.
While legal issues might technically be possible, there seems little point in it.
I'm not a lawyer and do not give legal advice, but in academic culture, nobody reasonable will be upset with you if you decide you no longer want a job because you need to take care of someone else.
Dear Future Supervisor,
I regret to inform you that I will not be able to take up the position of Postdoc in your group. I have recently learned that I must care for a family member instead. I am sorry for the inconvenience and I wish you the best of luck filling the position.
HR might ask you to sign something. If they do, only sign it if you are sure you understand it and agree to it.
Do not rush. You signed the contract, at the time you were perfectly fit for the proposed job. Now, between the signing of the contract and the official start of the employment, you had some event of major cause that may prevent you doing the job.
The event was not under your control, so you have good ground to cancel the contract (there is probation time, there is always the issue of visa to be sorted out). I would proceed carefully, completing the visa procedure and at the same time asking for a posticipation of starting date.
Under the employment contract'terms, there may be special conditions to assist close relatives falling ill, or having disabilities (temporary or permanent) needing special and constant supervision.
First things first: be clear and transparent with your future supervisor, call him/her and tell him/her what you told us here, be as detailed as you feel like but as precise as possible. What would be your "dream" plan B? working from remote part-time, assisting your relative? does it make sense for such a plan (were you going to take over a lab position? or something else that can be done remotely?).
Good luck, and all the best for your family.
What field? You might be able to “save” the postdoc by working remotely from your home country if you feel up to caring for your family member and doing the postdoc work at the same time. No US legal consequences can reach you, wherever you are since those kinds of treaties don’t exist for failure to fulfill a contract. If you feel up to it, try asking. Worst case is that some day you move to the US and get sued for them having to spend time finding a different person. That’s a loser of a case for them as it will cost them more than they could get out of you, and it’s not illegal for one private party to fail to hold up their end of a contract. That’s what lawsuits are for.