He severed links with his sister as she had repeatedly spoken badly about his husband.

If you're using "as" to mean "because", I would recommend changing it to this for clarity (she spoke badly, then he severed links).

I'm in awe of his braveness!

"courage" is more common than "braveness"

My sister-in-law has seemed unhinged since her Covid infection,; maybe these areis is a long-term consequences thereof.

"thereof" ties your second phrase to the first one
; is necessary because "maybe these are long-term consequences" is a sentence in itself, not a dependent phrase
I would use the singular "this is a long-term consequence" simply because it sounds more natural