I'm starting to panic..

Don’t forget that ( hopefully) your daughter will also have the support of her DHs family and friends.

Be supportive and start saving to go and visit, on the other hand they might just be looking at all the options.

Hmm this whole plan to move sounds a bit scatterbrained and illogical for the reasons given. Are you sure they weren't just daydreaming out loud? I bet it comes to nothing.

AbsolutelyGrandma70s

I’m a bit puzzled about the apparent assumption that American secondary schools would be any better!

MiniMama - best to keep a distance, don’t bring the subject up yourself and try not to worry, it may never happen.
Just offer your daughter the support you have no doubt been providing all her life.

OP’s daughter and sil are only ‘considering’ the move. As Grannyactivist and others are pointing out there are many aspects to assess.

It seems slightly strange to want to move continents because their catchment secondary school is naff - most people just move up the road.

Also, the quality of schools varies over time, depending to a large degree on the head, who may have changed in the next 6 years.

All good points from grannyactivist I think you should talk about the health care issues given your daughter's medical history. As your son in law is American I'm sure he will have given this some thought.
Like all countries there are high crime areas, anyone not in the UK would think we are all knife wielding maniacs so get this in perspective.
My daughter married an American as well and went to live there. They had a great life with their children in USA until sadly he died.
Don't put unnecessary doubts in your daughter's mind they could have a great life there.

A very valid point made by grannyactivist, the problem with healthcare in the US is not a lack of it, it is that insurance is ridiculously high and there in no real concept of universal healthcare like you have in the UK. If your son has a good job in the US waiting for him, with a strong health packet thrown in, it might be a great thing for them.

I think the crux of this is to get as much reassurance as you can. So firstly I would ask where in the States they're going to live? There are lovely low crime areas all over America, check out where they plan to live and if it's not a 'safe' area perhaps you could discuss this with your daughter. America has a system that delivers amazing health care (including mental health) for those who have good insurance, so maybe check out whether your son-in-law will have work-related insurance and if not, how they propose to pay for it. Does the neighbourhood they propose to go to have a good community and will your son-in-law's family be nearby to offer family support?
If you can be reassured then it will be easier all round to let them go and your questions will perhaps help them to clarify their own thinking as long as you can appear fairly neutral.