I have to sympathise with your poor SIL, OP.
I did a bit of research on scientific articles about Long Covid recently, as I have had three concussions over the past decade and there has been recently research into how LC can be similar in presentation to concussion patients, which is leading medical professionals to work on treatment that is similar to post concussion syndrome patients.
Having lived with post concussion syndrome for a year after my first concussion a decade ago, I can see how awful it is to live with Long Covid, especially if it is probably caused by inflammation, as a head injury is not dissimilar too in some symptoms (cognitive etc).
I was terrified of getting Covid and hoping that I'd avoided it but recently went through a week of it during the summer holidays, and survived. I didn't have the vaccination as I also have a medication induced neurological involuntary movement disorder called tardive dyskinesia that was caused by some off label antipsychotic after my head injury, and I'm extremely sensitive to medication side effects now.
I can absolutely understand how your SIL feels, especially when considering how debilitating my post concussion syndrome symptoms were. One of my friends has Long Covid too, and she's only just gone back to work in the past year
But, and i say this with kindness, she needs to go out and live life without fear. It's easier said than done of course, and I do totally empathise with the anxiety. I've been there, but know it's nowhere near as bad as it could be. It is obviously very hard for her, but she has to go out and feel her fear while just making a life for herself. She will see her life pass her h6 otherwise and that would be a real shame.😔