anxiety by one member of the family

I want to focus on your son. Did he go to see his brother and then go home as normal, did he see his brother and then stay away from the family home so his wife felt safe? Did he choose not to go?
You say you are not 'allowed' to visit them? I take it they have not visited you either? Do you do video calling? I'm sure you've been in touch in other ways as we all have had to do.
I think you have to leave your son to sort this one.

A few days ago I might have thought that your daughter in law was a bit over anxious, but having seen the lack of social distancing going on in our area now that the restrictions have been relaxed, I am inclined to think that maybe she is the one that is right and the rest of us are being a bit over confident.
See how she is after a few more weeks, if the rates keep going down then maybe do as MOnica suggests, but if the rates look like leading to a third wave, maybe she is just being sensible.

No, it is not coercive control. It is one exceptionally anxious woman trying to protect her family.

This is a medical/psychological problem and should be tretaed as such. Help needs to come, ideally through her husband who should talk to her gently about her fears, why is she so anxious about the dangers of COVID when numbers of cases are plummetting and so many people have now been vaccinated? What kinds of re-assurances would she require before she is able to relax?

Do any of you know why she should be so over-anxious. Has something happened during the pandemic or before that frightened her and triggered her current anxiety?

You do not say how old the children are. Are they school age? Are they back at school.

There are a number of websites with help for those getting anxious during the current emergency and I am sure later posrers will be able to recommend some.

If all else fails she may need medical treatment and should be encouraged to see her GP who may want to prescribe approrpiate treatment or recommend councilling.

The one thing to remember is that this is not coercive control or anything like it. Do not go down that road. It is a very anxious and fearful woman desperately trying to protect her family, whose fears have got out of control and who may need medical treatment of some sort.